{collector,collection} were once reserved in order to indicate different
runtime logical extent instance of multi-reference pclusters.
However, de-duplicated decompression has been landed in a more flexable
way, thus `struct z_erofs_collection` was formally removed in commit
87ca34a7065d ("erofs: get rid of `struct z_erofs_collection'").
Let's handle the remaining leftovers, for example:
`z_erofs_collector_begin` => `z_erofs_pcluster_begin`
`z_erofs_collector_end` => `z_erofs_pcluster_end`
as well as some comments. No logic changes.
Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx>