compiler: don't warn on -package-name for now

This avoids the annoying conundrum explained in #9956 - for the 7.10
release, we'll keep (silently) supporting this flag so -Werror doesn't
trip anything up.

Really, we could just say 'deal with it' to users probably, but the fix
is painless and does remove a sharp corner.

Signed-off-by: Austin Seipp <austin@well-typed.com>

Reviewers: ezyang

Differential Revision: https://phabricator.haskell.org/D742

GHC Trac Issues: #9956