std: Fix search-and-replace typos
rustpkg: Extract version number from git, as per #5684

For now, the test I added just checks that PkgId::new parses the
version number out of a git repo's tags list, where relevant.
rustpkg: Accept package IDs like github.com/foo/bar#0.3

If the package ID is of the form s#v, where v is a valid version
string, fetch tag v of that package.
auto merge of #6904 : catamorphism/rust/rustpkg_version_vcs, r=brson

r? @brson