More JVM backend cleanup (#25747)

Part of #25218

- Like the rest of the compiler, use `Context` in methods, do not
capture it inside classes
- Includes splitting `CoreBTypes` into `BTypeLoader` and
`WellKnownBTypes`; the latter is what `CoreBTypesFromSymbols` used to
be, minus loading stuff that went into the former. Only the latter
captures a Context (and it's the only remaining class to do so).
  - This means we can stop modifying (!) the `Context` in `GenBCode`
- We also no longer need to lock while calling `genClassDef` (it isn't
supposed to be multi-threaded anyway, but right now on main if you
remove the lock all hell breaks loose)
- Remove remaining JVM backend references from the frontend.
- Simplify code, delete dead code, etc.

We can't quite remove `frontendLock` / PPFA yet, as this happens if we
do:
```
[info] Test dotty.tools.dotc.CompilationTests.parallelBackend started
[                                        ] completed (0/7, 0 failed, 1s)Error while emitting gopher/OChannel
Cyclic reference involving class LambdaDeserialize

 Run with -explain-cyclic for more details.
Compilation failed for: 'compiling 'tests/pos/i10477' in test 'parallelBackend''
[=================================>      ] completed (6/7, 1 failed, 4s)Error while emitting adts/t2$Option$
Cyclic reference involving class LambdaMetafactory

 Run with -explain-cyclic for more details.
[=================================>      ] completed (6/7, 1 failed, 4s)Compilation failed for: 'compiling 'tests/pos/reference' in test 'parallelBackend''
[=======================================>] completed (7/7, 2 failed, 4s)
```

## How much have you relied on LLM-based tools in this contribution?

not

## How was the solution tested?

existing tests, this is a pure refactoring

Author: SolalPirelli