Track in the skykey whether the ast we're requesting is for the prelude

ASTFileLookupValue:
- The Key no longer inherits AbstractSkyKey. In the future it will be made an abstract class, similar to BzlLoadValue.Key.

ASTFileLookupFunction:
- Added stub comment for adonovan's upcoming change to make loads bind globally.
- Tightened an arg type.

BzlLoadFunction:
- Use ast keys, not labels, as the keys of the ast cache. (In pathlogical builds, the same label may appear as both a prelude label and an ordinary .bzl. Anyway this is cleaner.)

BzlLoadValue:
- Add Key#getASTKey(), to encapsulate creating the appropriate ast key from a bzl key.

Work toward #11437 and toward flat Starlark environments.

RELNOTES: None
PiperOrigin-RevId: 325092884