Allow specifying custom package-versions in package list

[snoyberg]

I think I described this requirement elsewhere, and it may already be implemented (in which case, we can close it). Motivation: add an additional package not found in LTS/Nightly, and/or tweak a single package. Functionality would be:

• Unpack that to somewhere like $DIR/.stack/extra-packages/foo-1.2.3
• Treat it like any other local dependency

[snoyberg]

Should be implemented, passing to @DanBurton for adding a test.

[snoyberg]

Closing, we'll need to go through the full feature list and determine missing tests later