Warn when depending on files targets in a pex_binary or python_awslambda

[Eric-Arellano]

One of our most common reported gotchas is confusion why files targets are not showing up with pex_binary. #11551 attempted to fix this by simply including the files, but it turned out much more complex than I expected, including:

1. Requiring the user to set the binary to unzip or venv mode (not the default), and
2. Using __file__ to anchor the relpath, which behaves differently between ./pants package and ./pants run.

While we may still want to allow including files if we can address the above, this improves the status quo. It corresponds with new docs warnings:

• https://www.pantsbuild.org/v2.0/docs/resources#files
• https://www.pantsbuild.org/v2.0/docs/python-package-goal#creating-a-pex-file-from-a-pex_binary-target
• https://www.pantsbuild.org/v2.0/docs/awslambda-python#step-2-define-a-python_awslambda-target

Example warning:

The pex_binary target src/py/project transitively depends on the below files targets, but Pants will not include them in the PEX. Filesystem APIs like open() are not able to load files within the binary itself; instead, they read from the current working directory.

Instead, use resources targets or wrap this pex_binary in an archive. See https://www.pantsbuild.org/v2.4/docs/resources.

Files targets dependencies: ['assets/f.txt:files', 'assets:relocated']

[ci skip-rust]