Check that sdist exports are limited to relevant files

tests/sdist/create/other-file.txt

@@ -0,0 +1 @@
+This file will not be in the source distribution!

tests/sdist/sdist_test.py

@@ -22,3 +22,6 @@ def test_import_sdist_archive(dds: DDS):
     assert not repo_content_path.is_dir()
     dds.repo_import(dds.build_dir / 'foo@1.2.3.tar.gz')
     assert repo_content_path.is_dir()
+    assert repo_content_path.joinpath('library.jsonc').is_file()
+    # Excluded file will not be in the sdist:
+    assert not repo_content_path.joinpath('other-file.txt').is_file()