Experiemental build env backend by pypa/build

The current build backend is moved to a subpackage, and the new one can
be enabled with --use-feature=build.

Author: uranusjr

src/pip/_internal/build_env.py

@@ -0,0 +1,22 @@
+"""Build Environment used for isolation during sdist building.
+
+Three implementations are available:
+
+* NoOpBuildEnvironment: Does not actually do anything, used by legacy code.
+* CustomBuildEnvironment: Old build environment implemented by pip developers.
+* PyPABuildEnvironment: New build environment that uses pypa/build.
+"""
+
+from .base import NoOpBuildEnvironment, get_runnable_pip
+from .custom import BuildEnvironment as CustomBuildEnvironment
+from .pypa import BuildEnvironment as PyPABuildEnvironment
+
+BuildEnvironment = NoOpBuildEnvironment
+
+__all__ = [
+    "BuildEnvironment",
+    "CustomBuildEnvironment",
+    "NoOpBuildEnvironment",
+    "PyPABuildEnvironment",
+    "get_runnable_pip",
+]