bazel syntax: make Eval private

Make Eval private. It should never have been public.
Mostly it was used as a hook for the debugger (via subclassing).
Now the debugger installs a hook directly into a global variable
that Eval inspects before executing each statement.

Make all package-visible functions of Eval static.
It should become the sole locus of knowledge about the tree-walking evaluator.
A follow-up change will move all tree-based evaluation logic from
Expressions into this class (Statements are done already).

Environment.evaluate(String) has been split into an expression flavor
(debugEval) and a file flavor (debugExec). If we eliminate the
pointless Debuggable interface these can become static methods in
EvalUtils, in a follow-up.
This is a behavior change to the debugger: entering "x=1; x+1"
will execute x+1 only for its effects; its value, as a statement,
is now None, not 2.

Delete DebugServerUtils.
Combine its install and remove methods as setDebugger, and move it to EvalUtils.

Delete DebugCallable (unused).
Delete DebugAwareEval (obsolete).

PiperOrigin-RevId: 268941210