In this change we:
- stop treating the nsPluginDirServiceProvider as a directory provider, as its
  GetFile implementation was a no-op anyway - registering it didn't make any
  difference.
- stop treating it as a class entirely, because the PLID getters were already
  static, so instantiating it also didn't do anything.
- move IO from the plugin directory list provider and the Windows-only PLID
  getters into nsPluginHost. This enables us to move it off of the main thread
  later - the directory getting has to happen on the main thread, but we can
  postpone further checks on the nsIFile instances.
- in the process, stop doing exists() calls on files because we can fail more
  lazily. This allows us to remove more allowlist entries from
  browser_startup_mainthreadio, though the `isDirectory` calls will actually
  still cause IO - they don't seem to create IO markers in the profiler.
  We will move this IO away from the main thread in subsequent commits.

Depends on D48328

Differential Revision: https://phabricator.services.mozilla.com/D48329

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extra : moz-landing-system : lando