[mypyc] Backport cached-group Extension.depends fix from upstream

[georgesittas]

Problem

The cross-group header-deps work that shipped in 2.1.0.post1 was upstreamed and merged (squash commit ab8e4bf1305416372ea70076193a766800f26410), but the improvements added during review never flowed back to release-2.1. The release branch only carries the original pre-review commits.

The missing piece matters in practice: when a group is fully cached, mypyc returns its cfile name with empty contents. The per-cfile loop then calls get_header_deps on the empty string, so Extension.depends ends up empty for every cached group. The existing fallback (749ff715c) only covers groups that return no cfile entries at all, which is not the shape that occurs under pip's multi-invocation build.

With empty depends, setuptools' newer_group check never sees changed cross-group export-table headers, and stale consumer .o files get linked against rebuilt dependencies whose struct layout shifted. This is the bug behind the sqlglot CI segfault: a cached sqlglotc wheel shipped a stale qualify.o whose quote_identifiers export-table slot dispatched into qualify_outputs after _add_ilike_columns was inserted mid-struct in qualify_columns' export table.

Fix (verbatim backport of the review-added hunks)

• mypyc_build: when ctext is empty for a cached group, re-read the on-disk .c before computing header deps.
• get_header_deps: assert non-empty contents so this can't regress silently.
• fudge_dir_mtimes: stop shifting linker outputs back. Combined with write_file's +1s bump, the old behavior made every .c permanently newer than its .so, forcing unconditional rebuilds that masked depends bugs in the test harness.
• Add the testIncrementalCrossGroupExportTableOffsets regression test.

Verification

fudge_dir_mtimes skip	build.py fix	result
no	no	passes (bug masked)
yes	no	fails (wrong class returned from cross-group call)
yes	yes	passes

All 3 test variants pass, test_misc.py 8/8, and the incremental/cross-module/separate run-test slice passes 97/97.