warnings: Find out if a qualified name is in the interactive scope directly

There were two ad-hoc mechanisms used to determine which modules were in
the interactive scope.

1. Look at everything in the GRE, to see what is imported qualified.
2. Look at the last loaded module in the HPT.

(1) Is very inefficient, GlobalRdrEnvs can be very big.
(2) is incorrect, there is no reason to assume the "last" thing added to
the HPT has any relevance to module loading order.

Happily, the same checks can be implemented directly by looking at the
interactive imports from the interactive context. This mirrors what
happens for normal imports.

Arguably, the error reporting code shouldn't be doing this kind of
processing and it should be an option is set when rendering the error
message. However, this just improves the situation and doesn't block
progress on that front in future.

See #14225 and #15611

Fixes #25600