Implement NoFieldSelectors extension (ghc-proposals 160)

Fixes #5972. This adds an extension NoFieldSelectors to disable the generation
of selector functions corresponding to record fields.  When this extension is
enabled, record field selectors are not accessible as functions, but users are
still able to use them for record construction, pattern matching and updates.
See Note [NoFieldSelectors] in GHC.Rename.Env for details.

Defining the same field multiple times requires the DuplicateRecordFields
extension to be enabled, even when NoFieldSelectors is in use.

Along the way, this fixes the use of non-imported DuplicateRecordFields in GHCi
with -fimplicit-import-qualified (fixes #18729).

Moreover, it extends DisambiguateRecordFields to ignore non-fields when looking
up fields in record updates (fixes #18999), as described by
Note [DisambiguateRecordFields for updates].

Co-authored-by: Simon Hafner <hafnersimon@gmail.com>
Co-authored-by: Fumiaki Kinoshita <fumiexcel@gmail.com>