fix(util/lean_path): if directory 'foo' does not contain 'default.lean', then we should also check whether the file 'foo.lean' exists or not, fixes #102

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(util/lean_path): throws an exception if there is an ambiguity when importing a module

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>