[3.14] gh-69605: Check for already imported modules in PyREPL module completion

[ambv]

Cherry picked from commits 7db209b and aed9050.

• Issue: Readline completion of module names in import statements #69605

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot s390x Fedora Stable Refleaks 3.14 (tier-3) has failed when building commit 98a2556.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1776/builds/798) and take a look at the build logs.
4. Check if the failure is related to this commit (98a2556) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1776/builds/798

Failed tests:

• test_external_inspection

Failed subtests:

• test_only_active_thread - test.test_external_inspection.TestGetStackTrace.test_only_active_thread

Summary of the results of the build (if available):

==

Click to see traceback logs

Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.14.cstratak-fedora-stable-s390x.refleak/build/Lib/test/test_external_inspection.py", line 1246, in test_only_active_thread
    self.assertEqual(
    ~~~~~~~~~~~~~~~~^
        len(gil_traces), 1, "Should have exactly one GIL holder"
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
AssertionError: 0 != 1 : Should have exactly one GIL holder