lib: Move bareuseronly verification into commit/core

lib: Move bareuseronly verification into commit/core

Conceptually `ostree-repo-pull.c` should be be written using
just public APIs; we theoretically support building without HTTP
for people who just want to use the object store portion and
do their own fetching.

We have some nontrivial behaviors in the pull layer though; one
of those is the "bareuseronly" verification.  Make a new internal
API that accepts flags, move it into `commit.c`.  This
is prep for further work in changing object import to support
reflinks.

Closes: #1193
Approved by: jlebon