Fix config file search and inclusion in header

[chrysle]

Just stumbled over this issue again, so I thought I might as well fix it.

Closes #1902

Contributor checklist

• Included tests for the changes.
• PR title is short, clear, and ready to be included in the user-facing changelog.

Maintainer checklist

• Verified one of these labels is present: backwards incompatible, feature, enhancement, deprecation, bug, dependency, docs or skip-changelog as they determine changelog listing.
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[webknjaz]

Could you explain this? Obviously, there's some unspecified side effect in external logic, but I don't get it.

[chrysle]

This adds the current working directory to the search path for source files as well, as proposed in the linked issue.

[webknjaz]

Why is this needed?

[chrysle]

See #1902 (comment). Making only src_files eager doesn't work though, as it won't pick up the config if not specified, then.

[sirosen]

I'm going to pick this up and try to get it into shape to include in the 7.4.2 release. I think this is a basically good fix and just needs some cleanup.

If I understand #1902, this is going down one of the viable paths. --config is marked eager and sets a callback, and that determines order of execution within click. If we want to establish better control over the ordering, a refactor may be appropriate -- I have ideas but that would inflate the size of the fix, and they can wait.

At a minimum, this needs

• a rebase
• better comments

I'm also going to assess to see if we can do more in terms of the tests.

[sirosen]

After discussion, I'm moving this out of 7.4.2 to try to be very narrow in what extra work we include for a bugfix release. But I've assigned myself and plan to come back to it.