[core] Flatten grid packages folder

[cherniavskii]

Closes #10114

[mui-bot]

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Updated pages:

• docs/data/data-grid/components/components.md
• docs/data/data-grid/localization/localization.md

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[flaviendelangle]

You should be able to remove a ternary in the copyFiles.mjs file that was due to this nested folder structure

[flaviendelangle]

When I migrated to use the same bundling strategy as the core, this code you just removed was the only diff with the core file
So maybe you could just remove everything from this file and import the core one, like we are doing for buildTypes.mjs

But this need to be double checked of course

[cherniavskii]

I'll look into this and submit a follow-up PR if necessary

[cherniavskii]

Unfortunately, there are other non-trivial differences between our script and the one from the core repo.
Making the copyFiles script generic would be a significant effort and I think it's not worth it.
But I think it makes sense to at least reuse some functions from that script.
I'll submit a PR to the core repo.

[cherniavskii]

mui/material-ui#40970

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