Adding support for revision pinning in dependency version

In order to make another step towards more reproducible build plans, @hdgarrood, @tfausak and I talked about the possibility of specifying version strings like:

```
== 2.0.0.0-1
```
where the `-1` specifies a revision.

This asks some questions like "What should be the behaviour when the package repo doesn't have such a revision?", and I think this also questions the way we declare dependencies nowadays (but that's for another time).

cc @emilypi

Start by reviewing the proposed dependency syntax `== 2.0.0.0-1` and the unresolved questions in this issue, especially behavior when a package repository lacks the requested revision. The issue names no files, tests, or entry points, so completion criteria would need to be established before implementation.

Written by the indexing model from the issue text.