Allow class to import a mixin with the same selector (Issue #136)

A class could already import a mxin with the same name but if another
selector within the same scope as that class and the identically-named
mixin tried to import the mixin, a stack overflow would occur. See Issue
#136 for more details and a reproduce case (or see the new unit test in
this changeset).

Although GitHub for Windows showed only the changed lines when I
had to leave a commit message, after sync'ing it would appear that it's
changed all of the line returns and so every line appears as changed.
Using the ?w=1 hack on github.com shows just the changes that I made
but if this is not acceptable then I can try to recreate the changes (though
considering I only changed the affected lines I'm not sure what I could do
better, it's line the Windows client cloned the repo with the Windows-style
line returns.. please let me know if I'm missing something and could
produce a better quality pull request).