Rename core.repo-finders to core.default-repo-finders

This renames a config key to make its semantics more obvious. Despite
what the commit message says, it only applies when a set of repo finders
is not specified (either on the command line or in a library API call).
This also renames the corresponding ostree_repo_get function. We can do
this since it hasn't been released yet.

Closes: #1763
Approved by: pwithnall

src/libostree/ostree-repo.h

@@ -113,7 +113,7 @@ gboolean      ostree_repo_set_collection_id (OstreeRepo   *self,
                                              GError      **error);
 
 _OSTREE_PUBLIC
-const gchar * const * ostree_repo_get_repo_finders (OstreeRepo   *self);
+const gchar * const * ostree_repo_get_default_repo_finders (OstreeRepo   *self);
 
 _OSTREE_PUBLIC
 GFile *       ostree_repo_get_path (OstreeRepo  *self);