In order to support both the `build_meta` and `build_meta_legacy`
backends, the core functionality is wrapped in a class with methods to
be overridden in build_meta_legacy. The class is an implementation
detail and should remain private.

Failing test adapted from PR pypa#1643

Co-authored-by: Tzu-ping Chung <[email protected]>

This runs all build_meta tests, plus a test that it is possible to
import from the directory containing `setup.py` when using the
build_meta_legacy backend.

This is part of the solution to GH pypa#1642, it is a
backwards-compatibility backend that can be used as a default PEP 517
backend for projects that use setuptools but haven't opted in to
build_meta.

Per Nick Coghlan's suggestion on PR pypa#1652, a try/finally block ensures
that the path is restored even in the event of an error.

Using the absolute path to the directory of the setup script better
mimics the semantics of a direct invocation of python setup.py.

Rather than exposing a top-level module for the legacy backend, we will
move the legacy backend into the `setuptools.build_meta` module and
specify it using the module:object syntax.