Remove 'metadata' and 'category' parsing from pkgmgr_process_* functions

Remove 'metadata' and 'category' parsing from pkgmgr_process_* functions

 - pkgmgr_process_* functions will no longer call any plugin,
 - pkgmgr_parse_* functions will follow old behaviour,
 - export __metadata_t and __category_t structures for app-installer plugin loading,
 - export enumaration for metadata plugin call.

Following changes should be submitted together:
 - https://review.tizen.org/gerrit/#/c/59029/
 - https://review.tizen.org/gerrit/#/c/59028/
 - https://review.tizen.org/gerrit/#/c/59030/
 - https://review.tizen.org/gerrit/#/c/59031/

Change-Id: Ifdb2acd914077ce8e3fb21b6117d1c192f5834e5