Always build libtorchcodec{ffmpeg_major}.so instead of libtorchcodec.so

[NicolasHug]

When BUILD_AGAINST_INSTALLED_FFMPEG is set, we build against a single FFmpeg version, and create libtorchcodec.so. With this PR, we now create libtorchcodec{installed_ffmpeg_major_version}.so instead.

Pros:

• we know from the .so name which ffmpeg version we're using.
• the names of the .so files are consistent whether BUILD_AGAINST_INSTALLED_FFMPEG was set or not.

Cons:

• The Cmake implementation is NASTY
• The logic of finding the installed FFmpeg version must be duplicated across Python and Cmake. This is prone to de-synchronizations and bugs.

IMHO the cons outweigh the pros by a lot. I'd be in favour of not landing this, and removing the associated TODOs.

This PR addresses this comment: https://www.internalfb.com/diff/D58527965?dst_version_fbid=262903956885891&transaction_fbid=1932366733863261

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@NicolasHug merged this pull request in 8030065.