Bug 1906026 - Make dependentlibs.py fail when it doesn't find any dependencies. r=firefox-build-system-reviewers,nalexander

Until such a time where libxul doesn't have dependencies at all, this is
not supposed to happen. When it does, it's because the output from the
tool used to produce the list changed, or the tool is broken is some
other way.

Differential Revision: https://phabricator.services.mozilla.com/D215582

toolkit/library/build/dependentlibs.py

@@ -132,6 +132,8 @@ def gen_list(output, lib):
 
     deps = dependentlibs(lib, libpaths, func)
     base_lib = mozpath.basename(lib)
+    if not deps:
+        raise RuntimeError(f"Couldn't find any dependencies of {base_lib}")
     deps[base_lib] = mozpath.join(libpaths[0], base_lib)
     output.write("\n".join(deps.keys()) + "\n")