std: Add a file-renaming function to std::os
rustpkg: Add a not_a_workspace condition
rustpkg: Allow package directories to appear in the RUST_PATH

This commit adds a rustpkg flag, --rust-path-hack, that allows
rustpkg to *search* inside package directories if they appear in
the RUST_PATH, while *building* libraries and executables into a
different target directory.

This behavior is hidden behind a flag because I believe we only
want to support it temporarily, to make it easier to port servo to
rustpkg.

This commit also includes a fix for how rustpkg fetches sources
from git repositories -- it uses a temporary directory as the target
when invoking `git clone`, then moves that directory into the workspace
if the clone was successful. (The old behavior was that when the
`git clone` failed, the empty target directory would be left lying
around anyway.)