pkg_mgmt, Support for git-hosted modules specified with custom revisions &/or tags

[luketpeterson]

Currently a module dependency in a PkgInfo struct or a GitCatalog can be specified by git URL and branch. However, there is no way to specify a tag or revision.

Cargo's handling of git-hosted crates is described here: https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#choice-of-commit

For additional reading, this issue is a continuation of the discussion here:
#691 (comment)