babel-loader: Fix a few issues with config caching (#83973)
- `isReactCompilerRequired` needs to be called in `getCacheCharacteristics`, not in `getFreshConfig` because it depends on the file contents, so we don't want to cache the result of `isReactCompilerRequired`.
- Moving react compiler detection outside of the cache lets us avoid using the full file path for a cache key (which negates much of the benefit of the cache).
- `getFreshConfig` shouldn't use the real file path or source map, those should always be injected later.
- It doesn't look like we were updating `inputSourceMap` on cached config objects (!?!?)
