recover Url.is_wellformed from before d8330439823a, e.g. relevant for JEdit_Resources.read_file_content (the URI alone does not necessarily have a protocol prefix, so plain file-path would be treated as URL);
src/Pure
src/FOL
src/HOL
src/ZF
src/CCL
src/CTT
src/Cube
src/FOLP
src/LCF
src/Sequents
src/Doc
src/Tools