Filter out build requirements that require an extra to be used

There is no mechanism provided for build requirements to have extras.

It should be acceptable to enforce that any "optional" packages
that are supposed to be conditionally installed based on the presence of
an extra should not be installed in a build environment.

Author: pradyunsg

src/pip/_internal/build_env.py

@@ -175,8 +175,10 @@ def check_requirements(
             )
             for req_str in reqs:
                 req = Requirement(req_str)
-                if req.marker is not None and not req.marker.evaluate():
-                    continue  # FIXME: Consider extras?
+                # We're explicitly evaluating with an empty extra value, since build
+                # environments are not provided any mechanism to select specific extras.
+                if req.marker is not None and not req.marker.evaluate({"extra": ""}):
+                    continue
                 dist = env.get_distribution(req.name)
                 if not dist:
                     missing.add(req_str)

tests/functional/test_build_env.py

@@ -178,6 +178,7 @@ def test_build_env_requirements_check(script: PipTestEnvironment) -> None:
             [
                 "bar==2.0; python_version < '3.0'",
                 "bar==3.0; python_version >= '3.0'",
+                "foo==4.0; extra == 'dev'",
             ],
         )
         assert r == (set(), set()), repr(r)