Introduce -Wcompat-unqualified-imports

This implements the warning proposed in option (B) of the Data.List.singleton CLC discussion. This warning, which is included in -Wcompat is intended to help users identify imports of modules that will change incompatibly in future GHC releases. This currently only includes Data.List due to the expected specialisation and addition of Data.List.singleton.

Fixes #17244 (closed).