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Revert using libCST for dep inference due to performance (#10907) (#11001)

Unfortunately, libCST resulted in a major slowdown.

Before, ff02ef0dd:

```
▶ /usr/bin/time ./pants --no-pantsd typecheck src/python/pants::
...
       10.63 real        11.47 user         6.99 sys
```

After, ea542ac16:

```
▶ /usr/bin/time ./pants --no-pantsd typecheck src/python/pants::
...
       35.40 real        36.13 user         7.08 sys
```

This makes sense: libCST stores a Concrete Syntax Tree, rather than an AST. It's doing more work, like preserving whitespace.

Unfortunately, this means that you must again run Pants with Python 3.8 in order to parse Python 3.8-only syntax. Because the original fix results in a 240% slowdown, this is less offensive than the slowdown, even though it's not ideal. 

[ci skip-rust]
[ci skip-build-wheels]

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