fix (moPepGen): cyclic-import

[zhuchcn]

Description

Cyclic import is fixed. I moved a lot of constant variables to a separate module which resolved most of the issue. And then certain imports that are only needed for type notation, I moved them under TYPE_CHECKING.

Adding Nick because you were involved in the earlier discussion.

Closes #838

Checklist

• This PR does NOT contain PHI or germline genetic data. A repo may need to be deleted if such data is uploaded. Disclosing PHI is a major problem.
• This PR does NOT contain molecular files, compressed files, output files such as images (e.g. .png, .jpeg), .pdf, .RData, .xlsx, .doc, .ppt, or other non-plain-text files. To automatically exclude such files using a .gitignore file, see here for example.
• I have read the code review guidelines and the code review best practice on GitHub check-list.
• The name of the branch is meaningful and well formatted following the standards, using [AD_username (or 5 letters of AD if AD is too long)]-[brief_description_of_branch].
• I have added the major changes included in this pull request to the CHANGELOG.md under the next release version or unreleased, and updated the date.
• All test cases passed locally.

[lydiayliu]

Best for @nwiltsie to comment! Seems like you made the lintr happy so that's good with me lol

[nwiltsie]

I don't know too much about the internals here, but the general form of pulling constants into a separate file seems good to me. I tried using pydeps --show-cycles to visualize the dependency changes, but unfortunately the output was incorrect.

As long as pylint is satisfied and the tool still works, I'm happy!

[zhuchcn]

Thank you for your comment! I'm merging it now. @nwiltsie