[FR] Support specifying MANIFEST.in config via pyproject.toml

What's the problem this feature will solve?

Now that setuptools has support for specifying setup.cfg options in pyproject.toml, one config file remains: MANIFEST.in. It would be nice if MANIFEST.in options could be specified in pyproject.toml so that a setuptools project could be configured entirely through one file.

(And before anyone suggests it, setuptools_scm does not help me: there are files in source control that I don't want to include in sdists, like .gitignore and .github/, so I'd still need a MANIFEST.in, and I'm back to square one.)

Describe the solution you'd like

Just spitballing: the following MANIFEST.in:

graft tests
global-exclude *.py[cod]

could be represented in pyproject.toml as:

[tool.setuptools]
manifest = [
    "graft tests",
    "global-exclude *.py[cod]",
]

or maybe:

[tool.setuptools]
manifest = [
    ["graft", "tests"],
    ["global-exclude", "*.py[cod]"],
]

Alternative Solutions

No response

Additional context

No response

Code of Conduct

• I agree to follow the PSF Code of Conduct

Start by reviewing how setuptools currently reads configuration from pyproject.toml and how MANIFEST.in directives are parsed. Compare the proposed string-list and nested-list representations, then identify the configuration and manifest-processing entry points that would need coverage. Done means projects can express the relevant MANIFEST.in directives in pyproject.toml without losing existing behavior.