Syntax to let Nimble know where a dependency exists relative to package file path
This is a remix of @Araq's idea. I think it would be useful for packages that want to consist of multiple packages and refer to them via their ``.nimble`` file.

Example:

```nim
requires "jester@:./deps/jester"
```

Syntax is up for discussion but hopefully you get the idea. In that case Nimble would simply add ``./deps/jester`` to the Nim path and wouldn't install the Jester package at all. This is in fact a form of vendoring.

Let me know your thoughts.