Move NoOpBuildEnvironment to its own file

This makes it easier to notice that there's three implementations of
build environment management.

src/pip/_internal/build_env/__init__.py

@@ -3,11 +3,8 @@
 
 from typing import Literal
 
-from pip._internal.build_env._base import (
-    BuildEnvironment,
-    NoOpBuildEnvironment,
-    get_runnable_pip,
-)
+from pip._internal.build_env._base import BuildEnvironment, get_runnable_pip
 from pip._internal.build_env._custom import CustomBuildEnvironment
+from pip._internal.build_env.noop import NoOpBuildEnvironment
 
 BuildIsolationMode = Literal["noop", "custom", "venv"]

src/pip/_internal/build_env/_base.py

@@ -128,31 +128,3 @@ def install_requirements(
         kind: str,
     ) -> None:
         raise NotImplementedError()
-
-
-# This class is needed since you can't initialise a base class.
-class NoOpBuildEnvironment(BuildEnvironment):
-    """A build environment that does nothing."""
-
-    lib_dirs: List[str] = []
-
-    def __enter__(self) -> None:
-        return
-
-    def __exit__(
-        self,
-        exc_type: Optional[Type[BaseException]],
-        exc_val: Optional[BaseException],
-        exc_tb: Optional[TracebackType],
-    ) -> None:
-        return
-
-    def install_requirements(
-        self,
-        finder: "PackageFinder",
-        requirements: Iterable[str],
-        prefix_as_string: str,
-        *,
-        kind: str,
-    ) -> None:
-        raise NotImplementedError("This should never get called.")

src/pip/_internal/build_env/noop.py

@@ -0,0 +1,32 @@
+from types import TracebackType
+from typing import List, Optional, Type, Iterable
+
+from pip._internal.build_env import BuildEnvironment
+from pip._internal.index.package_finder import PackageFinder
+
+
+class NoOpBuildEnvironment(BuildEnvironment):
+    """A build environment that does nothing."""
+
+    lib_dirs: List[str] = []
+
+    def __enter__(self) -> None:
+        return
+
+    def __exit__(
+        self,
+        exc_type: Optional[Type[BaseException]],
+        exc_val: Optional[BaseException],
+        exc_tb: Optional[TracebackType],
+    ) -> None:
+        return
+
+    def install_requirements(
+        self,
+        finder: "PackageFinder",
+        requirements: Iterable[str],
+        prefix_as_string: str,
+        *,
+        kind: str,
+    ) -> None:
+        raise NotImplementedError("This should never get called.")