[Python] fixed path being ignored in GeneratePythonGRPC

[tasansal]

Hi,

The Python gRPC code generator was ignoring -o <output_dir> option in flatc. With this, it generates all the services within the <output_dir> and under the correct namespace(s), i.e., Python sub-modules.

I had to remove the custom logic and use BaseGenerator's file prefix logic, which seems to work as expected.

I believe this closes #7397.

Thanks.

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[mustiikhalil]

@dbaileychess I think this should also be included in the next release. It fixes a bug I introduced in #6456

[tasansal]

Rebased to main so it's up to date

[jarkenau]

@dbaileychess friendly ping, would be great to get this merged