Fix bug where ${CABAL_PACKAGE}.cabal may not exist.

The CABAL_PACKAGE environment variable holds the Debianized name of the
Haskell package (e.g., for the X11-xft Haskell package, CABAL_PACKAGE
will be 'x11-xft').

This means that ${CABAL_PACKAGE}.cabal may not exist. Try
glob("*.cabal") instead.