Replace "unalias" with "expand_alias" to make debugging easier

Since "unalias" is already a shell built-in, having the lunar code full of "unalias" makes reading the source code and debugging a confusing and annoying experience.  So I decided to fix it by replacing "unalias" with "expand_alias".

This will probably break modules that (ab-)use "unalias" in their build scripts, but they probably shouldn't be doing that anyway.