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).
Merge pull request #339 from ProductiveRage/master

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