Fix AMP warnings for explicit Prelude imports (#8004)

No AMP warnings will be issued anymore when the name is not imported
from Prelude anymore. For example, a local definition of 'join' is now
legal in modules containing 'import Prelude (map)' for example. This
allows better future-proofing of libraries.

See also http://ghc.haskell.org/trac/ghc/ticket/8004#comment:16

Authored-by: David Luposchainsky <dluposchainsky@gmail.com>
Signed-off-by: Austin Seipp <austin@well-typed.com>