Improve the error for duplicate package names

[RaphGL]

If you make a package with the same name as an imported one, it gives you this error:

raph@archlinux ~/Projects/TermCL % odin check sdl3 -no-entry-point
/usr/lib/odin/vendor/sdl3/sdl3__foreign.odin(1:2) Error: Duplicate declaration of 'package sdl3' 
	package sdl3 
	^~~~~~~~~~^ 
	A package name must be unique 
	There is no relation between a package name and the directory that contains it, so they can be completely different 
	A package name is required for link name prefixing to have a consistent ABI 
/home/raph/Projects/TermCL/sdl3/backend.odin(1:2) found at previous location

This only points to the original (or one of the?) package with the name, this makes it hard for tooling like OLS to give nicer errors.
The error points to my file that caused the issue but since it's buried in the error message OLS can't know. If you're only looking at local diagnostics, it would appear like there were no errors at all.

So my suggestion is to issue a new error for each duplicate, so that tools that rely on odin itself for errors can catch it.
For context, here's the original issue I created on OLS: DanielGavin/ols#1039