[6.x] Fix cache misses for namespaced and subfolder fieldsets

[daun]

FieldsetRepository::find() keeps fieldsets in memory, but two kinds of handles never hit the cache:

• Namespaced handles like vendor::name. The lookup uses Arr::get(), which turns : into .. So foo::bar is looked up as foo..bar, but it is stored as foo::bar.
• Slash handles like sub/test. The lookup runs before / is normalized to ., while the store uses sub.test.

On a miss, the YAML file is read and parsed again. Blueprints call find() for every import: and every field: handle.field reference, so blueprints that import addon fieldsets re-parse them on every request. An example would be global set blueprints, which the cascade builds on every frontend request. In a custom addon, a global blueprint importing 12 namespaced fieldsets caused about 13 YAML parses per page request.

Changes

• find() normalizes the handle, then looks it up as a plain array key
• Once the cache actually works, entries can go stale, so we make sure that
  • save() stores the saved fieldset in the cache
  • delete() removes it
  • reset() removes it

Tests

• Same instance is returned for plain, dot, slash and namespaced handles, including namespaced subfolders
• Cache is correct after save(), delete() and reset()

Related

#13013 also adds cache updates to save()/delete(). The two PRs might conflict slightly there.