rustpkg: Clean up usage messages for install
rustpkg: Handle non-numeric versions; some cleanup

Package IDs can now be of the form a/b/c#FOO, where (if a/b/c is
a git repository) FOO is any tag in the repository. Non-numeric
tags only match against package IDs with the same tag, and aren't
compared linearly like numeric versions.

While I was at it, refactored the code that calls `git clone`,
and segregated build output properly for different packages.
rustpkg: Don't assume a non-numeric refspec is a tag

Just pass it directly to git, without prefixing it with tags/
docs: Talk about tags that aren't versions in the "Package identifiers" section
auto merge of #8032 : catamorphism/rust/rustpkg-tags, r=graydon

r? @graydon    Package IDs can now be of the form a/b/c#FOO, where (if a/b/c is
    a git repository) FOO is any tag in the repository. Non-numeric
    tags only match against package IDs with the same tag, and aren't
    compared linearly like numeric versions.
    
 While I was at it, refactored the code that calls `git clone`,  and segregated build output properly for different packages.