Temporary variables in make .for loops

Hello fellow porters.

My variable modifiers are growing and become too incomprehensible, so
here is the question I wanted to ask long ago:

How do I make and use a temporary variable in the .for loop?

This is what I have in Mk/Uses/cabal.mk:

.  for package in ${_use_cabal}
_PKG_GROUP=        ${package:C/[\.-]//g}

MASTER_SITES+=    https://hackage.haskell.org/package/:${package:C/[\.-]//g}
.  endfor

I'd like to write ${_PKG_GROUP} instead of repeating that regexp
incantation, but due to specifics of .for operation it doesn't work.
Using := with _PKG_GROUP also doesn't fix the problem.

Now I need to add another regexp substitution to every expression
inside this loop and it'll become even less readable than before.

Is there a clean solution to this problem?

Thanks in advance.