Imported grammars are not validated for file name matching the grammar name

In reviewing [this test](https://github.com/antlr/antlr4/blob/37fb7a71fbb5a8f820644c977419ef3506e23141/tool/test/org/antlr/v4/test/rt/java/TestCompositeParsers.java#L136), it seems the slave grammar **T.g4** contains an invalid header (for grammar `S`). A test should be added which verifies imported grammars meet the same requirements as top-level grammars for file/parser name consistency.

Start with tool/test/org/antlr/v4/test/rt/java/TestCompositeParsers.java at the linked test and inspect how the imported grammar T.g4 is handled. Add coverage showing imported grammars enforce file/parser name consistency like top-level grammars; done means the test fails for the invalid header and passes for valid imported grammar names.

Written by the indexing model from the issue text.