Check for cabal-install source-files earlier in build.
Check for cabal-install source-files earlier in build.

Signed-off-by: Edward Z. Yang <dbd597f5635f432486c5d365e9bb585b3eaa1853@cs.stanford.edu>

themoritz/cabal,headprogrammingczar/cabal,headprogrammingczar/cabal,kolmodin/cabal,mydaum/cabal,Heather/cabal,mydaum/cabal,sopvop/cabal,thomie/cabal,Heather/cabal,gbaz/cabal,kolmodin/cabal,bennofs/cabal,sopvop/cabal,themoritz/cabal,themoritz/cabal,thomie/cabal,gbaz/cabal,bennofs/cabal,sopvop/cabal