Feature request: allow a more general use of `[sources]` for dependencies

[giannipetrella]

Hello,

The documentation of the [sources] section of Project.toml files currently states the following:

[Dependencies listed via [sources] are] not used if [the current] project is a package that is being used as a dependency.

I would like to do precisely that, i.e., to use an unregistered package A as a dependency for another unregistered package B, which I plan to share with third parties who would only use B.

The Project.toml file for Package B would ideally look like

name = "B"
uuid = "BBBBBBBBBBBBBBBBB"
authors = ["me"]
version = "1.0.0"

[deps]
A = "AAAAAAAAAAAAAAAAA"

[sources]
A = {url = "https://github.com/the-repository-hosting-A", rev = "some-version"} 

Consequently, I would ideally just instruct folks wishing to use B to install it via the one-liner

julia> using Pkg; Pkg.add(url = "https://github.com/the-repository-hosting-B", rev = "some-version")

To handle versioning (i.e., to know what version of A is being installed when B is added) a choice of rev could for instance be required, or a warning could be thrown if there isn't one.

This would greatly simplify the workflow of people who develop niche packages with niche dependencies (e.g. for research purposes), do not want to go through the hassle of setting up a dedicated registry and do not want their users to have to add said registry.