Support multiple haskell packages in the same repo for DISABLE_SDIST_GIT_CHECK option

[harendra-kumar]

We can have per project .packcheck.ignore files and each project can ignore the other project directories. The .packcheck.ignore file can reside in the same directory as the .cabal file.

[harendra-kumar]

We perhaps do not need to anything in packcheck to support this. If we have each project in its own directory which contains a .packcheck.ignore, packcheck just needs to list git files under that dir and compare it against the .packcheck.ignore file which it probably already does.