erofs: avoid obsolete {collector,collection} terms

{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
87ca34a7 ("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.

Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230817082813.81180-2-hsiangkao@linux.alibaba.com