mk/haskell.mk: only add PLIST_SUBST and PRINT_PLIST_AWK if useful

The PLIST_SUBST and PLIST_PRINT_AWK definitions for Haskell library
packages are only useful if the package-description file exists.  If
that file is absent though, these are skipped.

The test whether the file exists is made as late as possible since that
file does not yet exist at the point where the package Makefile is
parsed.

This also affects the show-all-haskell target, which only shows these
values after the install phase.  This is not perfect but good enough for
practical cases.

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