I seem to have some fundamental "flaw" in portage.  It seems I am not 
able to write an ebuild that will in effect be able to replace another 
one but with a different name.

With RPMs, no matter how the RPM is named, it has "provides" data in it. 
  Is there some similar mechanism in portage?  It seems to me that if 
the name of an ebuild is changed, then *all* ebuilds depending on it 
will have to change too.  That looks like a PITA to me if it's true.

For example, if I have an overlay that provides alternative/altered 
packages of already existing ones in the portage tree, they will "clash" 
with portage.  Let's assume that my overlay provides an ebuild called 
"foo-alt" which is a variation of a package in portage called "foo", but 
is totally compatible with it.  What I'm looking for is being able to 
emerge "foo-alt", but have the ebuild state clearly that it provides the 
"foo" dependency, so ebuilds depending on "foo" will be satisfied if 
"foo-alt" is installed but "foo" isn't.

Possible?