Fix issues with path mapping in map_each functions

[fmeum]

Fixes the following issues with path mapping applied to map_each functions:

• Action keys could be stale if an action implementation switches from hard-coded artifact strings to Files mapped at execution time. This commit introduces a special PathMapper instance for fingerprinting, see the long comment on PathMappers.FOR_FINGERPRINTING. It includes a number of tests to verify that neither too much nor too little is mapped during action key computation.
• The Starlark objects representing mapped artifact roots didn't have the correct semantics for equality and ordered comparisons. They are now unequal to unmapped roots.
• When not using path mapping, the StarlarkSemantics of a thread used to evaluate map_each callbacks is not modified.

These changes require splitting PathMappers out of analysis_cluster and also make StrippingPathMapper a subclass of PathMapper instead of using an anonymous one.