fix(backend): resolve dotted backend object paths

[repzakske]

build-backend = "build.api:holder.backend" is valid under PEP 517, whose object_path grammar permits multiple attribute components. Currently the backend performs one getattr with the whole object path, so startup fails with AttributeError: module 'build.api' has no attribute 'holder.backend'.

Resolve the object path one component at a time. Extend the existing subprocess integration test with nested and deeply nested backend objects, retain the existing single-object cases, and assert the exact returned requirements. The two nested cases fail on unmodified main and pass with this fix.

Validation on Windows:

• Full suite: 83 tests pass, with 100% line and branch coverage, on Python 3.11.16, 3.14.8, and 3.15.0 (using coverage's C tracer).
• pre-commit run --all-files --show-diff-on-failure passes.
• ty check --output-format concise --error-on-warning . passes.

[gaborbernat]

Superseded by #319, which keeps your commit and adds test ids. This branch doesn't allow maintainer edits, so I couldn't push there. Thanks!