distutils-r1.eclass: Implement PEP517 mode

Add a PEP517 mode to the distutils-r1.eclass in order to facilitate
building packages via PEP517 backends.  In order to use it, set
DISTUTILS_USE_PEP517 to the appropriate build system name.  The eclass
will take care of setting BDEPEND, then invoke the backend to build
a wheel and then install its contents in python_compile().  The install
phase is limited to merging the staging area into the image directory.

In PEP517 mode, the test phase is automatically provided with venv-style
install tree that should suffice the vast majority of test suites.
As a result, distutils_install_for_testing should no longer be necessary
and is not available in this mode.

The new mode can also be used to install pre-PEP517 distutils
and setuptools packages.  To do so, just specify setuptools backend.
If pyproject.toml is missing, the eclass assumes legacy setuptools
backend that invokes setup.py.  It also enables setuptools-vendored
distutils, effectively carrying the migration from deprecated stdlib
version.

The PEP517 support effectively deprecates the legacy eclass mode.
This follows upstream deprecation of distutils and install commands
in setuptools.

Closes: https://github.com/gentoo/gentoo/pull/23750
Signed-off-by: Michał Górny <mgorny@gentoo.org>

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