Running `nimble build` on a project does not run `before install` on dependencies
Consider: nimbletest.nimble
```
version = "0.1.0"
author = "anonymous"
description = "something"
license = "MIT"

bin = @["nimbletest"]

requires "https://github.com/genotrance/nimtess2.git"
```

nimbletest.nim
```
import nimtess2/tesselator
```

When you run `nimble build` on this project, it attempts to install nimtess2 but doesn't run the `before install` step where nimgen is installed and run on the wrapper. As a result, this package does not build.

```
> nimble build

  Verifying dependencies for a@0.1.0
 Installing https://github.com/genotrance/nimtess2.git@>= 0.1.0
Downloading https://github.com/genotrance/nimtess2.git using git
  Verifying dependencies for nimtess2@0.1.0
      Info: Dependency on nimgen@>= 0.4.0 already satisfied
  Verifying dependencies for nimgen@0.4.0
      Info: Dependency on c2nim@>= 0.9.13 already satisfied
   Warning: Package 'c2nim' has an incorrect structure. The top level of the package source directory should contain at most one module, named 'c2nim.nim', but a file named 'cpp.nim' was found. This will be an error in the future.
      Hint: If this is the primary source file in the package, rename it to 'c2nim.nim'. If it's a source file required by the main module, or if it is one of several modules exposed by 'c2nim', then move it into a 'c2nimpkg/' subdirectory. If it's a test file or otherwise not required to build the the package 'c2nim.nim', prevent its installation by adding `skipFiles = @["cpp.nim"]` to the .nimble file. See https://github.com/nim-lang/nimble#libraries for more info.
  Verifying dependencies for c2nim@#head
      Info: Dependency on compiler@>= 0.16.0 already satisfied
  Verifying dependencies for compiler@#head
      Info: Dependency on regex@>= 0.7.1 already satisfied
  Verifying dependencies for regex@0.7.3
      Info: Dependency on unicodedb@>= 0.5.1 & < 0.6 already satisfied
  Verifying dependencies for unicodedb@0.5.1
      Info: Dependency on unicodeplus@>= 0.3.0 & < 0.4 already satisfied
  Verifying dependencies for unicodeplus@0.3.2
      Info: Dependency on unicodedb@>= 0.4 & < 0.6 already satisfied
  Verifying dependencies for unicodedb@0.5.1
 Installing nimtess2@0.1.0
   Success: nimtess2 installed successfully.
   Building a/a using c backend
       Tip: 40 messages have been suppressed, use --verbose to show them.
     Error: Build failed for package: a
        ... Details:
        ... Execution failed with exit code 1
        ... Command: "/home/genotrance/.nimble/bin/nim" c --noBabelPath --path:"/home/genotrance/.nimble/pkgs/nimgen-0.4.0"  --path:"/home/genotrance/.nimble/pkgs/c2nim-#head"  --path:"/home/genotrance/.nimble/pkgs/compiler-#head"  --path:"/home/genotrance/.nimble/pkgs/regex-0.7.3"  --path:"/home/genotrance/.nimble/pkgs/unicodedb-0.5.1"  --path:"/home/genotrance/.nimble/pkgs/unicodeplus-0.3.2"  --path:"/home/genotrance/.nimble/pkgs/unicodedb-0.5.1"  --path:"/tmp/nimble_19103/githubcom_genotrancenimtess2git_0.1.0"  -o:"/home/genotrance/a/a" "/home/genotrance/a/a.nim"
        ... Output: Hint: used config file '/home/genotrance/programming/nimdevel/config/nim.cfg' [Conf]
        ... Hint: system [Processing]
        ... Hint: a [Processing]
        ... a.nim(1, 16) Error: cannot open file: nimtess2/tesselator
```

Note that if you do `nimble install` instead of `nimble build`, it works just fine.

```
> nimble install

  Verifying dependencies for a@0.1.0
 Installing https://github.com/genotrance/nimtess2.git@>= 0.1.0
Downloading https://github.com/genotrance/nimtess2.git using git
Downloading https://github.com/genotrance/nimgen using git
  Verifying dependencies for nimgen@0.4.0
      Info: Dependency on c2nim@>= 0.9.13 already satisfied
  Verifying dependencies for c2nim@#head
      Info: Dependency on compiler@>= 0.16.0 already satisfied
  Verifying dependencies for compiler@#head
      Info: Dependency on regex@>= 0.7.1 already satisfied
  Verifying dependencies for regex@0.7.3
      Info: Dependency on unicodedb@>= 0.5.1 & < 0.6 already satisfied
  Verifying dependencies for unicodedb@0.5.1
      Info: Dependency on unicodeplus@>= 0.3.0 & < 0.4 already satisfied
  Verifying dependencies for unicodeplus@0.3.2
      Info: Dependency on unicodedb@>= 0.4 & < 0.6 already satisfied
  Verifying dependencies for unicodedb@0.5.1
 Installing nimgen@0.4.0
   Building nimgen/nimgen using c backend
    Prompt: nimgen@0.4.0 already exists. Overwrite? -> [forced yes]
   Success: nimgen installed successfully.
Setting up Git repo               <====== nimgen actually being run in before install
Checking out artifacts
Processing nimtess2/Include/tesselator.h
Generating nimtess2/tesselator.nim
Processing nimtess2/tesselator.nim
  Verifying dependencies for nimtess2@0.1.0
      Info: Dependency on nimgen@>= 0.4.0 already satisfied
  Verifying dependencies for nimgen@0.4.0
      Info: Dependency on c2nim@>= 0.9.13 already satisfied
  Verifying dependencies for c2nim@#head
      Info: Dependency on compiler@>= 0.16.0 already satisfied
  Verifying dependencies for compiler@#head
      Info: Dependency on regex@>= 0.7.1 already satisfied
  Verifying dependencies for regex@0.7.3
      Info: Dependency on unicodedb@>= 0.5.1 & < 0.6 already satisfied
  Verifying dependencies for unicodedb@0.5.1
      Info: Dependency on unicodeplus@>= 0.3.0 & < 0.4 already satisfied
  Verifying dependencies for unicodeplus@0.3.2
      Info: Dependency on unicodedb@>= 0.4 & < 0.6 already satisfied
  Verifying dependencies for unicodedb@0.5.1
 Installing nimtess2@0.1.0
   Success: nimtess2 installed successfully.
 Installing a@0.1.0
   Building a/a using c backend
   Success: a installed successfully.
```