Fix panic with plugins and build commands

This fixes a bug in cargo where target executables and libraries would be linked
to plugin native dependencies (not wanted!).

Closes #885
auto merge of #893 : alexcrichton/cargo/issue-885, r=brson

This fixes a bug in cargo where target executables and libraries would be linked
to plugin native dependencies (not wanted!).

Closes #885
Merge remote-tracking branch 'upstream/master'