fix: Quit panicking on named types over error

[prathshenoy]

A panic breaks indexing for any module whose package graph contains a defined type whose underlying type is the built-in error interface e.g., type CustomErr error. The crash fires during implementation-relationship extraction.

When scip-go walks a dependency's exported types and computes their method sets, the inherited Error method on a user-defined wrapper over error surfaces as a function whose enclosing package is nil but whose receiver has been rebound to the user-defined name CustomErr. The guard in GetSymbolOfObject recognized only the literal universe receiver, so the rebound form fell through to an unconditional panic.

This change unconditionally skips any function with a nil enclosing package during symbol lookup. The only such functions today are methods inherited from universe-block interfaces, which scip-go does not emit symbols for in any case, matching its existing behavior for direct Error calls on plain error-typed values.

[jupblb]

Thanks! 😃