Various fixes

- Convert function pointers to stored type when taking function value
- Handle invalid Return in unreachable blocks (Go Issue #7022)
- Use IsAFunction instead of IsGlobalConstant
- Ignore package "C" when computing dependencies in llgo-build
Fix runtime type generation for imported types
Don't define wrappers for external interfaces

They're defined at the origin... right? If not,
we'll need to do some kind of "One Definition Rule" type thing.
Define functions resolved during type mapping
Use "main.init" function, rather than generating proginit

Whoops, didn't know that was done for me.
Add unexported methods to runtime types

I think I've done this before. Added a comment.
Use correct package in promoted pointer method lookup
Implement more of runtime.eqtyp