Correctly preserve plugin deps

When copying files over from the old root to the new root on a fresh
compilation, care must be taken to preserve the correct plugin/host version of
each dependency. The previous code copied back over at most one library, but
this commit fixes this behavior by copying over all targets necessary for
compilation.
auto merge of #283 : alexcrichton/cargo/fix-fresh-plugin-deps, r=wycats

When copying files over from the old root to the new root on a fresh
compilation, care must be taken to preserve the correct plugin/host version of
each dependency. The previous code copied back over at most one library, but
this commit fixes this behavior by copying over all targets necessary for
compilation.