source_dir is always set when building a wheel or for editables

It is a remnant of the `setup.py install` era, and we can safely assume it is always set.

Author: sbidoul

src/pip/_internal/wheel_builder.py

@@ -50,8 +50,7 @@ def _should_build(
     if req.is_wheel:
         return False
 
-    if not req.source_dir:
-        return False
+    assert req.source_dir
 
     if req.editable:
         # we only build PEP 660 editable requirements

tests/unit/test_wheel_builder.py

@@ -63,8 +63,6 @@ class ReqMock:
             ReqMock(editable=True, use_pep517=True, supports_pyproject_editable=False),
             False,
         ),
-        # We don't build if there is no source dir (whatever that means!).
-        (ReqMock(source_dir=None), False),
         # By default (i.e. when binaries are allowed), VCS requirements
         # should be built in install mode.
         (