Ensure that project metadata is available.

        Under PEP 517, call the backend hook to prepare the metadata.
        Under legacy processing, call setup.py egg-info.

def prepare_metadata(self):
        # type: () -> None
        """Ensure that project metadata is available.

        Under PEP 517, call the backend hook to prepare the metadata.
        Under legacy processing, call setup.py egg-info.
        """
        assert self.source_dir

        with indent_log():
            if self.use_pep517:
                self.prepare_pep517_metadata()
            else:
                self.run_egg_info()

        if not self.req:
            if isinstance(parse_version(self.metadata["Version"]), Version):
                op = "=="
            else:
                op = "==="
            self.req = Requirement(
                "".join([
                    self.metadata["Name"],
                    op,
                    self.metadata["Version"],
                ])
            )
            self._correct_build_location()
        else:
            metadata_name = canonicalize_name(self.metadata["Name"])
            if canonicalize_name(self.req.name) != metadata_name:
                logger.warning(
                    'Generating metadata for package %s '
                    'produced metadata for project name %s. Fix your '
                    '#egg=%s fragments.',
                    self.name, metadata_name, self.name
                )
                self.req = Requirement(metadata_name)