In rustpkg, pass around sysroot; in rustpkg tests, set the sysroot
manually so that tests can find libcore and such.
With bonus metadata::filesearch refactoring to avoid copies.
The install command should work now, though it only installs
in-place (anything else has to wait until I implement RUST_PATH).
Also including:
core: Add remove_directory_recursive, change copy_file
Make copy_file preserve permissions, and add a remove_directory_recursive
function.
Pulled out tests into their own modules inside the files they test,
as per the draft style guidelines.
Started a new module, path_util, for utility functions to do with
paths and directories.
Changed default_dest_dir to use a condition and return Path
instead of Option<Path>.
