rustpkg: Implement URL-like package IDs

This patch implements package IDs like
github.com/catamorphism/test-pkg.

To support such package IDs, I changed the PkgId struct to contain
a LocalPath and a RemotePath field, where the RemotePath reflects
the actual URL and the LocalPath reflects the file name of the cached
copy. Right now, the only difference is that the local path doesn't
contain dashes, but this will change when we implement #6407.

Also, PkgIds now have a short_name field -- though the short name
can be derived from the LocalPath, I thought it was cleaner not to
call option::get() wantonly.
rustpkg: Tests for well-formed and ill-formed package IDs...

...and cleanup, making how we handle version numbers more rational
(specifically, not passing in a versioned name to rustc
with the -o flag), and removing unused code.
auto merge of #6418 : catamorphism/rust/rustpkg, r=catamorphism

r? @brson This patch implements package IDs like
github.com/catamorphism/test-pkg.

To support such package IDs, I changed the PkgId struct to contain
a LocalPath and a RemotePath field, where the RemotePath reflects
the actual URL and the LocalPath reflects the file name of the cached
copy. Right now, the only difference is that the local path doesn't
contain dashes, but this will change when we implement #6407.

Also, PkgIds now have a short_name field -- though the short name
can be derived from the LocalPath, I thought it was cleaner not to
call option::get() wantonly.