Merlin Framework: Improve "Failed to generate value mapper" error message

Checked for duplicates

Yes - I've already checked

Alternatives considered

Yes - and alternatives don't suffice

Related problems

When a directive type declares a field whose type does not have a value mapper, the annotation processor fails with this error at build time:

> Task :eurc:compileJava
/<snip>.java:55: error: Failed to generate value mapper for parameter
    public List<EquatorialCoordinate> targetList = List.of(EquatorialCoordinate.fromRadians(0, 0));

This does not adequately explain what the root problem is, or how to address it. In this case, the value mapper for EquatorialCoordinate is not registered under a @WithMappers annotation in this context. EquatorialCoordinate happens to be defined in a separate library, and similar uses within that library do work as expected, since its ValueMapper is known in that context. This makes it even less obvious at first glance what the problem is, since it appears to work sometimes but not others.

Describe the feature request

The given error message should be augmented to include the following information:

• The type(s, if multiple) for which a value mapper could not be found
• The package-info.java file for which the @WithMappers annotation(s) are expected to see a static factory for the given type's value mapper.

This will provide a much more direct path to a solution for a user who encounters this kind of problem for the first time (or in an unexpected context).

(For full disclosure, I'm reporting this issue on behalf of a user who raised a question revolving around this error in Slack.)

Strawman proposal:

/<snip>.java:55: error: Failed to generate value mapper for parameter -- no applicable ValueMapper factory for EquatorialCoordinate
    public List<EquatorialCoordinate> targetList = List.of(EquatorialCoordinate.fromRadians(0, 0));
/<snip>/package-info:.java:1: info: Has a value mapper for `EquatorialCoordinate` been registered at `/<snip/package-info:.java`?

(Repeat this pair of messages for every encountered type lacking an accessible ValueMapper.)

Start by locating the Merlin annotation processor code that emits “Failed to generate value mapper for parameter” and the related diagnostic tests, if present. Trace how the unavailable type and relevant package-info.java context are known. Done means diagnostics identify each missing type and the package-info.java location where its ValueMapper factory is expected.