#2135 does not allow multiple lexer imports within same file

#2135 does not allow multiple lexer imports within same file
Grammar:
```
options { tokenVocab=SomeLexer; }
```
Lexer:
```
lexer grammar SomeLexer;
import x;
import y;
```
Nor
```
lexer grammar SomeLexer;
import y;
```
Not permitted.

The issue names no files or tests. Reproduce the shown lexer grammars and trace the parser-generator handling of lexer imports; done means both single- and multiple-import forms are accepted with tokenVocab.

Written by the indexing model from the issue text.