Handle global packages in cabal-testsuite/Test/Cabal/Plan.hs JSON parser.
Handle global packages in cabal-testsuite/Test/Cabal/Plan.hs JSON parser.

Previously, parsing failed if the plan contained any non-local packages, because
they lacked the "dist-dir" field.

mydaum/cabal,themoritz/cabal,themoritz/cabal,mydaum/cabal,themoritz/cabal