x/tools/go/analysis: report informative error message about gob.Register(fact) conflicts

[adonovan]

gob.Register has an unfortunately unfixable bug whereby two packages registering types *p/a.T and *q/a.T get a panic because internally their names are both mapped to *a.T. Witness: https://go.dev/play/p/HtGLdqhHvaG

The analysis.Validate function has the opportunity to detect and report this confusing problem with a helpful explanation. Basically one of the types must be renamed.

@hyangah