* Removes unused usings.
* Eliminates unnecessary branches.
* Renames `NodeExecutorBase`'s override methods.

LibSass does not support relative paths resolution. Nor does ruby-sass
(unless we use Compass). At present, WE tries to resolve the URLs in
the post-compilation step by using one base path. This misleads in the
scenarios where we have nested imports and the meaning of
'relative path' need to be twisted to justify the outcome. :)

Less provides this functionality OOTB. We will have to wait for LibSass
to provide this functionality; either sass/libsass#674 or
sass/libsass#532 gets addressed;