This adds a new flag, -p, to %pyproject_buildrequires.
When set, the runtime dependencies are read from the pyproject.toml's
[project] table.
See: https://bugzilla.redhat.com/2261939
pyproject_buildrequires.py already had a short `-p` option for
--python3_pkgversion (hidden from the macro users).
This change removes the one-letter option and leaves the long-one.
`-p` is now reused for reading dependencies from pyproject.toml
and made visible to the macro users.
pyproject-srpm-macros is intended to be installed in the default buildroot.
That way, no explicit BuildRequires for pyproject-rpm-macros are required,
as long as %pyproject_buildrequires is used in %generate_buildrequires.
When only pyproject-srpm-macros is installed, the minimal implementation of
%pyproject_buildrequires generates a dependency on pyproject-rpm-macros.
When pyproject-rpm-macros is installed, it overrides the implementation
of %pyproject_buildrequires with the full one.
Note that in Fedora, pyproject-rpm-macros is required by python3-devel,
but not in RHEL.
This allows us to keep pyproject-rpm-macros in the RHEL CRB repository.
