What about saying that the copying step, if necessary, is part of the build backend's responsibilities? I.e. pip doesn't copy the whole directory to a temporary build location, but the build backend may decide to do that at its discretion when it's asked to build a wheel. pip would continue to handle this for setup.py builds.
That still leaves the other use cases for building sdists unsatisfied. In addition it’ll likely be pip that gets the bug reports when some backend inevitably doesn’t copy those files and then leaves random debris laying about (including things like read only file systems where random crap *can’t* be written to the ``.`` directory or mounting the same package in multiple docker containers that would cause different things to be pooped into the ``.`` directory).