Change namespace naming and aliasing for imports

The `.` character is invalid as a group name in regex, replacing by `__`.
The bug arose in `lexer._build_mres`.

lark/load_grammar.py

@@ -586,7 +586,7 @@ def import_from_grammar_into_namespace(grammar, namespace, aliases):
         try:
             return aliases[name].value
         except KeyError:
-            return '%s.%s' % (namespace, name)
+            return '%s__%s' % (namespace, name)
 
     to_import = list(bfs(aliases, rule_dependencies))
     for symbol in to_import:
@@ -745,7 +745,7 @@ class GrammarLoader:
                     g = import_grammar(grammar_path, base_paths=[base_path])
 
                 aliases_dict = dict(zip(names, aliases))
-                new_td, new_rd = import_from_grammar_into_namespace(g, '.'.join(dotted_path), aliases_dict)
+                new_td, new_rd = import_from_grammar_into_namespace(g, '__'.join(dotted_path), aliases_dict)
 
                 term_defs += new_td
                 rule_defs += new_rd

tests/grammars/ab.lark

@@ -0,0 +1,10 @@
+startab: expr
+
+expr: A B
+    | A expr B
+
+A: "a"
+B: "b"
+
+%import common.WS
+%ignore WS

tests/test_parser.py

@@ -1025,6 +1025,37 @@ def _make_parser_test(LEXER, PARSER):
             self.assertEqual(x.children, ['12', 'lions'])
 
 
+        def test_relative_rule_import(self):
+            l = _Lark_open('test_relative_rule_import.lark', rel_to=__file__)
+            x = l.parse('xaabby')
+            self.assertEqual(x.children, [
+                'x',
+                Tree('expr', ['a', Tree('expr', ['a', 'b']), 'b']),
+                'y'])
+
+
+        def test_relative_rule_import_drop_ignore(self):
+            # %ignore rules are dropped on import
+            l = _Lark_open('test_relative_rule_import_drop_ignore.lark',
+                           rel_to=__file__)
+            self.assertRaises((ParseError, UnexpectedInput),
+                              l.parse, 'xa abby')
+
+
+        def test_relative_rule_import_subrule(self):
+            l = _Lark_open('test_relative_rule_import_subrule.lark',
+                           rel_to=__file__)
+            x = l.parse('xaabby')
+            self.assertEqual(x.children, [
+                'x',
+                Tree('startab', [
+                    Tree('grammars__ab__expr', [
+                        'a', Tree('grammars__ab__expr', ['a', 'b']), 'b',
+                    ]),
+                ]),
+                'y'])
+
+
         def test_multi_import(self):
             grammar = """
             start: NUMBER WORD

tests/test_relative_rule_import.lark

@@ -0,0 +1,7 @@
+start: X expr Y
+
+X: "x"
+Y: "y"
+
+%import .grammars.ab.expr
+

tests/test_relative_rule_import_drop_ignore.lark

@@ -0,0 +1,7 @@
+start: X expr Y
+
+X: "x"
+Y: "y"
+
+%import .grammars.ab.expr
+

tests/test_relative_rule_import_subrule.lark

@@ -0,0 +1,7 @@
+start: X startab Y
+
+X: "x"
+Y: "y"
+
+%import .grammars.ab.startab
+