[SAoC] Implement -import-local

[ahmetsait]

This is currently a very crude PR, but should be pretty easy to review so don't hesitate to do so. I will be iterating this as I figure out how to refactor it for the better and to address reviews.
See info on -import-local switch here: SAoC Milestone Overview
See also the accompanying PR: #13115

[dlang-bot]

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#13235"