HaikuDepot : Rename Depot Url to Identifier

The old depot / repository "url" was renamed as
"identifier" some years ago. This field seemed
to remain as "url" and is now corrected.

Change-Id: Icd1390745311030e3fd57b852de91ed69b326496
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7190
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>

HaikuDepot : Break Out Pkg Model

The data model for packages contains a number
of components which all reside in a single
source header and implementation file making
maintenance and modifications difficult. This
change will break the sub-models out into
separate files without making any functional
changes.

Change-Id: I9c2557bc1e5a33661145c55290a0baad4084fd49
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7181
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>