Improve Hazel's handling of happy/alex template files

[thumphries]

From internal issue tracker:

The initial PR of merging cabal2build into Hazel has a workaround for happy and alex w.r.t. data files:
Quoting /pull/3 :

Note: this PR manually checks in the data-files that Happy and Alex use
and which are generated automatically by their custom Setup.hs scripts.
Figuring out a better solution is TODO; however, those files' interfaces
have been very stable, and upcoming releases of those packages
will include them, so I think this is OK for now.

Look for a less janky approach. For example:

• pier generates them manually by copying the logic in the Setup scripts
• Run the Setup script enough to build the package and then extract the data files