Generate .git_archival.txt for setuptools-scm packages built from source archives

[LalatenduMohanty]

Problem

Packages using setuptools-scm or flit_scm fail to build from source archives that lack .git metadata. Fromager creates stub PKG-INFO files as a fallback, but setuptools-scm checks .git_archival.txt before PKG-INFO in its version resolution chain.

Users can work around this by setting SETUPTOOLS_SCM_PRETEND_VERSION_FOR_{DIST} in per-package YAML (#946), but that requires manual configuration for every affected package.

Proposal

Add ensure_git_archival() — similar to the existing ensure_pkg_info() — to automatically generate a .git_archival.txt from the resolved version when one is missing or unprocessed:

node: 0000000000000000000000000000000000000000
node-date: 1970-01-01T00:00:00+00:00
describe-name: 1.2.3-0-g0000000000000000000000000000000000000000

setuptools-scm extracts the version from the describe-name field (tag-distance-gnode format from git describe). Distance 0 means exact tag match.

Scope:

• Applies only to source archives without .git (git clones don't need this)
• Preserves existing valid .git_archival.txt files
• Replaces unprocessed template files (containing $Format: placeholders)
• Must run before backend import to cover packages where setuptools-scm triggers during get_requires_for_build_wheel

Supersedes

• BUG: setuptools-scm/flit_scm failure before build_sdist: PKG-INFO ensured too late #960 — .git_archival.txt has higher priority than PKG-INFO in setuptools-scm's fallback chain, so generating it early enough eliminates the timing problem described there.

Related

• Give env package settings access to package version #946 — ${__version__} env var template (manual per-package alternative)