Support alternate ways of referring to packages in build-deps

[bitemyapp]

So that private packages are easier to work on.

With this thread in mind: https://twitter.com/Blaisorblade/status/543572310113153024

Does anyone have any ideas of what might be a nice way to better support users of Haskell on projects with private dependencies? git submodules and cabal sandbox add-source is too manual / script-y.

[23Skidoo]

Duplicate of #2189.

[23Skidoo]

git submodules and cabal sandbox add-source is too manual / script-y.

I think the situation could be improved by

1. Moving to a package tree-centric model (No package is an island #1367).
2. Adding a --pkgroot argument to add-source (or something like it) so that you could run a single command to add-source all dependencies from the current tree.
3. Fixing misc. usability issues with add-source deps (only reinstall the deps that the current package actually depends on, don't reinstall everything again when a single package failed, anything else I forgotten).