Mark field values from __defaults__ as being a default value.

[kaos]

Part 2 of 2 fixes #20947. (Part 1: #20949)

When providing default values using __defaults__ for Python source globs, they are now treated as a default value, rather than a explicit value. The implication is on how pants treats multiple globs for the two cases. For defaults, as long as any glob matches an existing file, all is well, where as for explicit globs, all of them must match existing files.

This was problematic when overriding the default source globs for the python_sources target for instance to make it recursive, as the defaults list many globs, some of which may not be applicable.

Now, the experience augmenting the default globs are seamless (no more unmatched globs issues):

# Make `python_sources` targets recursive by default.
__defaults__({
  python_sources: dict(
    sources = [
      "**/" + glob for glob in
      python_sources.sources.default   
    ]
  ),
  ...
})

[kaos]

Only 78fdb9f that needs reviewing in this PR. The rest comes from #20949.

[huonw]

Thanks for working on this.

Does this cause these values to be hidden from pants peek --exclude-defaults? Maybe that's desirable, but potentially is a behaviour change we should call out? Either that or change to have two levels of defaults (built-in vs user-supplied)?

[kaos]

Thanks for working on this.

Does this cause these values to be hidden from pants peek --exclude-defaults? Maybe that's desirable, but potentially is a behaviour change we should call out? Either that or change to have two levels of defaults (built-in vs user-supplied)?

Good call, I didn't think of this, but yea, as-is it will be excluded here! (nice! 😎) but I also think it's a good idea to be able to distinguish builtin defaults from user provided defaults. Perhaps as a new option that defines what peek considers being default for exclusion: builtin_defaults, user_defined_defaults or any_defaults.

[kaos]

As discussed on #20947 we opt to change the behavior for source globs instead of having a conditional on the value being default or not.