Locally developed packages are not taken into account when resolving http dependencies
Consider the following scenario:

1) You check out a project having a nimble file specifying some of its dependencies as a HTTP URLs.

2) You want to fix an issue in one of the HTTP dependencies, so you clone the this package locally and you run `nimble develop` to make it available system-wide. You fix the issue and you delete any previously downloaded version from your `~/.nimble/pkgs` folder.

3) You run `nimble test` in the original repo, cloned in step 1. At this point, Nimble doesn't recognize your local copy of the dependent package and instead downloads a fresh copy from the specified HTTP URL. Your local fixes are ignored in favor of the downloaded unmodified copy.

Expected outcome:

Nimble should be able to recognize that a locally linked version of a package (with `nimble develop`) satisfies a HTTP dependency for the same package.

Possible solutions:

1) The problem is that Nimble cannot figure out that a particular URL used as a dependency matches a locally installed package. One way to solve this is to introduce a local database for the installed packages which will keep a reverse "URL to package name" lookup table.

2) Nimble can download the HTTP dependency in a temporary location, then it can inspect the downloaded package name and figure out that it's already present as a `nimble develop` link.

3) The user may be required to specify the package name when introducing a HTTP dependency. The syntax may look like this:

``` nim
requires "nim >= 0.18.1",
         "httputils@https://github.com/status-im/nim-http-utils >= 0.2.0"
         # here, `httputils` is the package name
```

The specified package name will allow Nimble to check for an existing installation of the package. A similar solution is used by other build tools such as Scala's SBT. Alternatively, we can introduce some new properly typed dependency-specification API:

```
uses("eth_p2p", git = "https://github.com/p2p", branch = "master")
```

----------

I've pushed a very simple project demonstrating the issue here:

https://github.com/zah/nim-nimble-issue

To see the issue, you'll also need to clone the `httputils` package from https://github.com/status-im/nim-http-utils and then attempt to introduce changes in it after linking it with `nimble develop`.