As discussed in issue #378, when an embedded transformer (that is, one
passed to the Lark class using the transformer argument), is an
inplace transformer (either a subclass of Transformer_InPlace, or with
the @v_args(tree=True) decorator), the in-place transformer was not
working correctly and in-fact Lark used it like a normal non-in-place
transformer, expecting it to return the transformed value.

%import .local.foo -> bar

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

%import common.NUMBER -> N
%import .local.NUMBER -> N