Antlr 4.7 doesn't honor the -o <out_dir> option when generating Python3 parser code.

I have the following directory structure

grammar/ -- contains language.g4
lib/python/ -- location where the generated .py files are expected.

when executing antlr4 from within lib/python like so:

antlr4 -Dlanguage=Python3 -visitor -listener -o . ../../grammar/language.g4

the files get generated in the grammar directory.

Reproduce the issue with the shown antlr4 command, using grammar/language.g4 and the lib/python output location. Start by tracing the command-line handling of -o for the Python3 target. Done means the generated .py files are written under lib/python rather than grammar/.

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