release: Refactor dist set logic

Add to the METALOG immediately following the operation itself to make
the logic easier to follow. Fuse adjacent identical loops. Both of
these are now possible as we no longer have separate paths for NO_ROOT
builds.

Move MANIFEST out of the loops for disc1 and dvd1 and copy it
explicitly, so that all three MANIFEST operations are identical, and
shared by pkgbase and legacy code paths for disc1 and dvd1.

Reviewed by: jrm
Event: Kitchener-Waterloo Hackathon 202506
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50383