Register imported libraries in dependency order (#3620)

block_mark_referenced walks the joined block backwards and skips the
body of any definition not yet marked referenced, so a library must
precede its importers.  f58787c41 registered a library before walking
its dependencies, to have somewhere to keep the `loading` flag, which
reversed that order and made block_drop_unreferenced free definitions
that are still referenced through bound_by.

Keep the libraries being loaded on a stack threaded through the
load_library frames instead, so registration can move back after the
dependency walk.

tests/modules/deep3.jq

@@ -0,0 +1,2 @@
+def _f: 3;
+def f: _f;