Warn if user attempts to define a package with a reserved name.
Warn if user attempts to define a package with a reserved name.

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

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