[JENKINS-50336] Fix loading optional extensions after installing a missing dependency (#3370)

* [JENKINS-50336] Only keep extensions that could be loaded in memory

So that delta computations will re-attempt to load any extension that
previously failed or was skipped after a new plugin is installed.

After installing a plugin, need to refresh extensions in order to pick
up extensions that can be loaded thanks to this new plugin.

* [JENKINS-50336] @OptionalExtension annotated classes were not being picked up after dynamic load of variant

Recompute GuiceExtensionAnnotation list when refreshing extensions