If this is set for a remote we will never automatically look for
dependencies in it. This makes dependency search faster, as we
don't need to search in app-only remotes.
For now this is a purely client side setting which lets you store
the default branch to use for a remote. This is mostly meant to
be used for UI tools, although the CLI could also be made to use this.
You can set this manually in the CLI with --default-branch, or via a
flatpakrepo file (new key DefaultBranch).