Find builders anchored inside the referenced type #317

Part of #301.

Current state

The resolver only looks up a candidate builder class next to the referenced type (<Simple><builderUsageSuffix> in the same package). A builder that lives inside the referenced type itself — e.g. the common Lombok/Immutables pattern public static PersonBuilder builder() on Person — is never discovered, even though generated code could call it the same way MapStruct does.

Goal

Extend the builder search with a second candidate location, MapStruct DefaultBuilderProvider-style:

1. Look for an accessible static parameterless method on the referenced type (conventionally builder(), possibly a configurable name) returning a type that satisfies the builder contract.
2. Verify that returned type against the same capability checks as the same-package candidate (empty/prefilled instantiation, build()).
3. Decide precedence when both candidates exist (e.g. in-type builder() vs same-package XBuilder) — and whether an ambiguous in-type candidate (several matching methods) warns and skips like MapStruct.

Notes

• Keeps the contract checks from the sibling sub-tasks unchanged — only the lookup gains a second location.
• Worth deciding whether java.*/javax.* types are excluded from this lookup (MapStruct excludes them).