_tools/check_mod_exports.ts: modFilePath extracts file paths that are not strictly mod.ts files

[jbronder]

Describe the bug
In the process of adding a file named unstable_chmod.ts to the @std/fs package to add towards #6255, running deno task lint:mod-exportsthe _tools/check_mod_exports.ts script will interpret unstable_chmod.ts as a mod.ts file and will emit warnings that fail the lint task.

Steps to Reproduce

• Create a file that ends in *mod.ts, (e.g. fs/unstable_chmod.ts or fs/another_file_mod.ts) in the @std/fs package.
• Run deno task lint:mod-exports.

❯ ls fs/
_create_walk_entry.ts       empty_dir_test.ts           mod.ts
_get_file_info_type.ts      ensure_dir.ts               move.ts
_get_file_info_type_test.ts ensure_dir_test.ts          move_test.ts
_is_same_path.ts            ensure_file.ts              testdata
_is_same_path_test.ts       ensure_file_test.ts         unstable_chmod.ts  # <-- newly created file
_is_subdir.ts               ensure_link.ts              unstable_chmod_test.ts
_is_subdir_test.ts          ensure_link_test.ts         unstable_errors.d.ts
_map_error.ts               ensure_symlink.ts           unstable_errors.js
_to_file_info.ts            ensure_symlink_test.ts      unstable_lstat.ts
_to_path_string.ts          eol.ts                      unstable_lstat_test.ts
_utils.ts                   eol_test.ts                 unstable_stat.ts
copy.ts                     exists.ts                   unstable_stat_test.ts
copy_test.ts                exists_test.ts              unstable_types.ts
deno.json                   expand_glob.ts              walk.ts
empty_dir.ts                expand_glob_test.ts         walk_test.ts

❯ deno task lint:mod-exports # NOTE: absolute path not shown for brevity
Warn /Users/... .../deno_std/fs/unstable_chmod.ts does not export './eol.ts'.
Warn /Users/... .../deno_std/fs/unstable_chmod.ts does not export './empty_dir.ts'.
Warn /Users/... .../deno_std/fs/unstable_chmod.ts does not export './ensure_file.ts'.
Warn /Users/... .../deno_std/fs/unstable_chmod.ts does not export './ensure_dir.ts'.
Warn /Users/... .../deno_std/fs/unstable_chmod.ts does not export './expand_glob.ts'.
Warn /Users/... .../deno_std/fs/unstable_chmod.ts does not export './move.ts'.
Warn /Users/... .../deno_std/fs/unstable_chmod.ts does not export './ensure_symlink.ts'.
Warn /Users/... .../deno_std/fs/unstable_chmod.ts does not export './walk.ts'.
Warn /Users/... .../deno_std/fs/unstable_chmod.ts does not export './copy.ts'.
Warn /Users/... .../deno_std/fs/unstable_chmod.ts does not export './ensure_link.ts'.
Warn /Users/... .../deno_std/fs/unstable_chmod.ts does not export './exists.ts'.

Expected behavior
Expected the deno task lint:mod-exports to extract only mod.ts files.

Environment

• OS: MacOS 13.7
• deno version: 2.1.4
• std version: Reproduced with @std/fs v1.0.8