List goal with --changed-since doesn't list BUILD file changes

[KristianTashkov]

Describe the bug
Making a change in a BUILD file and then running on changed targets has differing behavior depending on whether you use --changed-since as a goal filter or if you save the result into a file via list and use --spec-files on the lint goal.

Steps to reproduce:

git clone https://github.com/pantsbuild/example-python
cd example-python
echo "" >> helloworld/BUILD

# This correctly fails black because the BUILD file format has an extra new line in the end
pants --changed-since=HEAD lint 

# runs linter only on source mapped inside changed BUILD FILE, but not the BUILD file itself so black passes.
pants --changed-since=HEAD list > changed_targets.txt 
pants --spec-files=changed_targets.txt lint   

With the contents of changed_targets.txt:

helloworld:pex_binary
helloworld/__init__.py:lib
helloworld/main.py:lib

Pants version
2.23.0

OS
MacOS

Additional info

[lilatomic]

The list goal:

Lists all targets matching the file or target arguments.

I think the list & --spec-files might be working as intended, since BUILD files are not targets. I'm actually a little surprised that pants --changed-since=HEAD lint includes the BUILD file. I thought it would only be linted with pants update-build-files --check. So maybe the --changed-since is incorrect.

[KristianTashkov]

Is there a reason there is no build_source target automatically generated for each BUILD file?
I agree that list specifically mentions outputting only targets, while --spec-files works with both targets and files, so the current behavior makes sense from a functional standpoint. From a practical standpoint we probably would want to have a way to run all relevant linters on changed (similar to how pants --changed-since=HEAD lint does it), but with a deferred saved list targets.

Keep in mind that pants lint :: also lints BUILD files, so update-build-files is not the only way to run linters on BUILD files currently.