…css/vite`

I am not 100% sure, but I believe this gets rid of at least 1 macroTask.

Otherwise it means that we:
1. Setup 2 compilers for an initial build
2. The second time we clear require caches which means that _everything_
   will be loaded from scratch.

During debugging I noticed that some of the files were in the list
multiple times, using a Set solves that issues. While this is a little
band-aid it helps for now. Following commits will include a more proper
fix.

During debugging, I noticed that every time the PostCSS code runs, the
full plugin re-runs. This means that caches are thrown away. I'm
currently not sure if this is a test specific issue or an actual issue.

That said, the integration tests do run postcss-cli for example directly
so I think this is a real issue.

This change should be safe since the cache is unique per input file
(which it was before as well). Worst case is that we use a bit more
memory.

Removing this means that the main file (e.g.: `tailwind.config.js`) is
only in the list once. This also means that we only have to register it
as a PostCSS dependency and it also means that we will `fs.stat` the
file once.

There’ve been some improvements in compatibility and perf in a few of them

…performance