fix(profiling): make explicitly marked main packages "my code" [backport 3.10]

[github-actions]

Backport 3fbe32f from #13649 to 3.10.

If a user runs their code as a "main module" which they installed like a
library (python -m my_program args...), and expicilty specifies that
it's the main package via DD_MAIN_PACKAGE, we should consider code from
that module "my code". Right now we don't, though, because we mark that
code as a "library" in the profiling code provenance info. Check
whether the user specified a main package and make sure it gets marked
as "my code" by leaving the kind blank.

Checklist

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• The change includes tests OR the PR description describes a testing strategy
• The PR description notes risks associated with the change, if any
• Newly-added code is easy to change
• The change follows the library release note guidelines
• The change includes or references documentation updates if necessary
• Backport labels are set (if applicable)

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• Testing strategy adequately addresses listed risks
• Newly-added code is easy to change
• Release note makes sense to a user of the library
• If necessary, author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• Backport labels are set in a manner that is consistent with the release branch maintenance policy

[github-actions]

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