Avoid symbol interning in `file_metadata`.

Avoid symbol interning in `file_metadata`.

This commit changes `created_files` so it uses strings directly as keys,
rather than symbols derived from the strings. This avoids the cost of
having to do the hash table lookups to produce the symbols from the
strings.

The commit also uses `entry` to avoid doing a repeated hash table lookup
(`get` + `insert`).

Note that PR #60467 improved this code somewhat; this is a further
improvement.