Fix #2051: Running unit tests from DUB single file packages fails

[drug007]

Performing test command dub imports every source file as module to build the project excluding the main source file. But this excluding may fail and in this case dub try to import the main source file as module. Single file package building fails because the main source file does not belong to any import path. Now both the current file and the main source file paths are compared in relative form to prevent importing the main source file as module.

[dlang-bot]

Thanks for your pull request and interest in making D better, @drug007! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

[drug007]

@thewilsonator I opened this PR instead of #2050 and it's ready to review. How to remove WIP label?
Would it be better if I'll split the commit to several like a commit with tests, a commit with changelog etc?