Assert scope is not match/guard when using escaped regexes

[sabiwara]

Following on #14720, I've been thinking if we could/should improve the error message when trying to use regex module attributes in guards/match scope.
This PR explores the idea, but feel free to close if this is overthinking on my part.

Before (confusing: :re.compile mentioned but not in the code)

After

[josevalim]

I think in this case some code duplication is fine. Let's just reimplement the error message directly in the Regex module, since that's what we would expect library authors to do too.

[josevalim]

Instead of generating additional code, we should call Regex.__import__(unquote(...)), where __import__ itself is a macro which also checks the environment.

[sabiwara]

This sounds much better indeed 👍

[sabiwara]

Arg, this might actually not work, since unlike Kernel we need to require Regex.

       quote do
            require Regex
            Regex.__import_pattern__(unquote(Macro.escape(exported)))
          end

Which then makes it impossible to compile elixir:

==> elixir (compile)
error: you are trying to use/import/require the module Regex which is currently being defined.

(although it seems weird to warn here, since it's happening within quote 🤔)

[sabiwara]

Pushed the broken commit just in case: c1dee89