[DNM] pkg/cdi: fixes and cleanups

[thaJeztah]

• stacked on pkg/cdi: fix, and cleanup some locks, and use RWMutex #345

pkg/cdi: simplify some code with slices, maps packages

Both slices.Clone and maps.Clone already handles nil values, so we can
skip our own checks, making the code slightly more straightforward.

Also update GetSpecDirErrors to make sure we perform the copy after
obtaining a lock.

pkg/cdi: clean up watcher lifecycle handling

• Stop the existing watcher before applying new cache options and resetting
  watcher-related errors, so the old watcher cannot observe partially updated
  configuration.
• Clear the watcher after closing so that we don't leave behind a stale,
  closed watcher.
• Combine watcher setup and startup so creating the fsnotify watcher,
  installing the initial directory watches, and starting the event loop happen
  as a single operation.
• Install the initial watches explicitly instead of routing startup through
  watch.update, and only initialize watch.tracked after successfully
  creating the watcher.
• Avoid starting the watch goroutine if creating the watcher failed.
• Remove the now-redundant nil check from watch.watch and use its fsw
  argument directly.

pkg/cdi: simplify refresh logic

All callers of refreshIfRequired, except Refresh, only use it to perform
an automatic refresh when necessary and ignore refresh errors. Refresh,
on the other hand, needs to force a refresh in manual mode and return the
cached errors regardless of whether a refresh took place.

Move the logic to decide between "automatic" or "forced" refresh to the
Refresh method, and move collecting the cached / refreshed errors to
that method. The responsibility of Cache.refresh is now reduced to
refreshing the data and errors, and it's up to callers to collect errors
when needed.

pkg/cdi: scope variables and move them closer to where used

pkg/cdi: ListClasses: combine loops

pkg/cdi: handle fsnotify operations as a bitmask

fsnotify.Event.Op is a bitmask and can contain multiple operations, but the
watcher compared it directly against individual operation values.

As a result, combined events could bypass operation-specific handling, such as
filtering writes and creates by file extension or detecting removal of a
watched directory.

Mask the operations we are interested in and test the individual bits instead.

pkg/cdi: ignore unrelated filesystem events

The watcher filtered write and create events by file extension, but rename
and remove events for unrelated files would still trigger a refresh of the
CDI cache.

Apply the same Spec-file filtering to all relevant filesystem events, while
still processing events for configured Spec directories themselves.

This avoids unnecessary rescans when non-Spec files in watched directories are
renamed or removed, without interfering with handling of removed Spec
directories.

WIP: cleanup / fix watch

[thaJeztah]

Let me just push the branch I had locally for my own sanity; I think I had some work remaining (last commit), but started to loose track of splitting out to small PRs (for easier review).

[thaJeztah]

Ah, right; I needed a go bump for some bits;

Error: pkg/cdi/cache.go:354:16: stdversion: slices.Sorted requires go1.23 or later (module is go1.21) (govet)
  	return slices.Sorted(maps.Keys(c.devices))
  	              ^
  Error: pkg/cdi/cache.go:365:16: stdversion: slices.Sorted requires go1.23 or later (module is go1.21) (govet)
  	return slices.Sorted(maps.Keys(c.specs))
  	              ^