Allow compiling with experimental API but without curl/soup

Allow compiling with experimental API but without curl/soup

This commit rearranges a few things in ostree-repo-pull.c so that OSTree
will successfully compile with experimental API enabled and without
libsoup, libcurl, or avahi:

./autogen.sh --enable-experimental-api --without-soup --without-curl
--without-avahi

This is accomplished with two sets of changes:

1. Move ostree_repo_resolve_keyring_for_collection() so it can be used
even without libsoup or libcurl.
2. Add stub functions for ostree_repo_find_remotes_async() and
ostree_repo_pull_from_remotes_async(), and their _finish() counterparts,
so they return an error when libsoup or libcurl isn't available.

Closes: #1605
Approved by: cgwalters