Don't treat trusted import as unknown type when referenced as struct member type

[borup3]

Description

Marks trusted imports as external symbols so --typecheck doesn't output Unknown type '%s' (assuming external C struct) when it doesn't have to assume.

This brings the compiler in line with the README during typecheck but with zero warnings:

This treats the header as a module and assumes all symbols accessed through it exist.

I reordered validate_types to avoid searching find_type_alias if it found a struct by that name, and chose to put is_extern_symbol before is_trait as I expect it to be a cheaper search for all projects (like find_selective_import), but as is_trait is more likely to return a match I can swap them back around if desired.

import "raylib.h" as rl;

struct ZImage {
    image: rl::Image;
}

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes