update-shape.py apply --accept-local scripts/repo_shape.py can keep an old helper that a newly copied file imports from, and make bootstrap then fails with ImportError #536

update-shape.py apply --accept-local scripts/repo_shape.py can keep an older shared helper that a file it copies in the same run needs.

• The case: a holder or project whose scripts/repo_shape.py is both (a local edit plus an upstream change) takes --accept-local for it. apply then copies the other upstream-changed files, which may import a name that only the new repo_shape.py has.
• The result is an ImportError the first time the copied script runs (make siblings, make bootstrap), while check reports unchanged and the validators pass.
• family.py: init refuses to land in the standard's checkout and names a runnable scripts/family.py, printed commands quote paths, and add, bump and remove refuse the tracking branch and print their branch's push (#488, #486, #487) #507 removed the one live instance (siblings.py no longer imports the new quoter), and rv456 met the same coupling for bootstrap.py (Both roots' scripts/bootstrap.py: main escapes what stdout cannot encode and spells each value it prints, so name: "a\ud83db" or 日本 under PYTHONIOENCODING=ascii no longer ends make bootstrap in a traceback (#391) #456). The general guard is still missing.
• Possible fix: when a copied file imports from scripts/repo_shape.py, apply checks that every name it imports exists in the kept copy, and refuses by name if not (or check reports it).

— #458, comment

#456 moves spelled() and escape_unencodable_stdout() into scripts/repo_shape.py, and both roots' bootstrap.py import them. A project that keeps a locally edited scripts/repo_shape.py (check reads it both) with apply --accept-local scripts/repo_shape.py, while taking the new bootstrap.py, gets an ImportError from scripts/bootstrap.py, --help and make bootstrap. Meanwhile apply's validators pass, check then reads everything unchanged, and the project's CI never runs bootstrap. Reproduced with a real scaffolded project. Following check's merge-by-hand avoids it. Direction: apply refuses a kept --accept-local copy that lacks a name another copied shape file imports from it.
— #458, comment