perf(build): use make -j option when invoking external makefile for compiling Lean libraries

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(util/lean_path): allow 'import dirname' as shorthand for 'import dirname.default'

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(util/lean_path): allow 'import dirname' as shorthand for 'import dirname.default'