Reduce operations.build.metadata's API to 1 function

Why: There isn't any state being maintained, or multiple uses of the
metadata generator for a single requirement. The additional complexity
does not help in any significant manner.

src/pip/_internal/operations/build/metadata.py

@@ -12,25 +12,20 @@
 from pip._internal.utils.typing import MYPY_CHECK_RUNNING
 
 if MYPY_CHECK_RUNNING:
-    from typing import Callable
-
     from pip._internal.req.req_install import InstallRequirement
 
 logger = logging.getLogger(__name__)
 
 
-def get_metadata_generator(install_req):
-    # type: (InstallRequirement) -> Callable[[InstallRequirement], str]
-    """Return a callable metadata generator for this InstallRequirement.
-
-    A metadata generator takes an InstallRequirement (install_req) as an input,
-    generates metadata via the appropriate process for that install_req and
-    returns the generated metadata directory.
+def generate_metadata(install_req):
+    # type: (InstallRequirement) -> str
+    """Generate metadata and return the metadata directory.
     """
+    func = _generate_metadata
     if not install_req.use_pep517:
-        return _generate_metadata_legacy
+        func = _generate_metadata_legacy
 
-    return _generate_metadata
+    return func(install_req)
 
 
 def _generate_metadata(install_req):

src/pip/_internal/req/req_install.py

@@ -24,7 +24,7 @@
 from pip._internal.exceptions import InstallationError
 from pip._internal.locations import distutils_scheme
 from pip._internal.models.link import Link
-from pip._internal.operations.build.metadata import get_metadata_generator
+from pip._internal.operations.build.metadata import generate_metadata
 from pip._internal.pyproject import load_pyproject_toml, make_pyproject_path
 from pip._internal.req.req_uninstall import UninstallPathSet
 from pip._internal.utils.compat import native_str
@@ -615,9 +615,8 @@ def prepare_metadata(self):
         """
         assert self.source_dir
 
-        metadata_generator = get_metadata_generator(self)
         with indent_log():
-            self.metadata_directory = metadata_generator(self)
+            self.metadata_directory = generate_metadata(req)
 
         if not self.name:
             self.move_to_correct_build_directory()