Add test case showing issue with isort for missing transient deps.

[kaos]

It seems there are cases even in fmt that requires transient deps to be present, not only for check.
See related comment: #14186 (comment)

I've run across this a couple of times now at work, where I have a pre-commit hook that runs on a subset of files (exactly the subset of changed files only), and fails, where as when I run by hand I usually use globs like a hammer and includes a whole lot more, which exposes this issue, due to isort not being stable for the two ways it is being invoked.

[kaos]

I found this isort option, which feels relevant, but I've not yet figured out how to use it to solve this issue (which I feel it ought to do): https://pycqa.github.io/isort/docs/configuration/options.html#src-paths

[thejcannon]

I think the correct route to take would be to use pants' knowledge of the python sources to seed its config.

[kaos]

I think the correct route to take would be to use pants' knowledge of the python sources to seed its config.

That would be great. I've not found out what config makes this work, though, besides including the first party sources being imported from the file being formatted.

I've played around with the src_paths config, and though I can see isort picking up a bunch of source files for that option (using ./pants fmt --only=isort --isort-args=--show-config), it still doesn't produce consistent output as I want it to.

[benjyw]

See my comment in #15069, but isort's statement in https://pycqa.github.io/isort/docs/configuration/options.html#src-paths that "modules within src paths have their imports automatically categorized as first_party" is not entirely true - there is a caveat for namespace packages... I propose a solution there.