checker: using only C functions from a module makes it think it's unused

[dy-tea]

Describe the bug

If you use only C functions from an imported module, the checker does not detect said module as being used. It gives the warning that the module is unused and should be used as _ to silence.
The import is definitely used because removing the import causes a failure in compilation.

Reproduction Steps

Create a project with the below files:

// main.v
module main

import cmod

fn main() {
	println(C.add(1, 2))
}

// cmod/test.h
int add(int a, int b) {
	return a + b;
}

// cmod/test.c.v
module cmod

#include "@VMODROOT/cmod/test.h"

fn C.add(a int, b int) int

Expected Behavior

Should not print any warnings.

Current Behavior

❯ v run .
main.v:3:8: warning: module 'cmod' is imported but never used. Use `import cmod as _`, to silence this warning, or just remove the unused import line
    1 | module main
    2 | 
    3 | import cmod
      |        ~~~~
    4 | 
    5 | fn main() {
3

Possible Solution

No response

Additional Information/Context

No response

V version

V 0.4.12 0b0f380

Environment details (OS name and version, etc.)

V full version	V 0.4.12 ba97785.0b0f380
OS	linux, "CachyOS"
Processor	12 cpus, 64bit, little endian, 13th Gen Intel(R) Core(TM) i5-1335U
Memory	2.69GB/15.31GB
V executable	/home/dylan/Repos/v/v
V last modified time	2025-12-29 19:31:41
V home dir	OK, value: /home/dylan/Repos/v
VMODULES	OK, value: /home/dylan/.vmodules
VTMP	OK, value: /tmp/v_1000
Current working dir	OK, value: /home/dylan/Repos/mod
Git version	git version 2.52.0
V git status	weekly.2025.52-79-g0b0f380e-dirty
.git/config present	true
cc version	cc (GCC) 15.2.1 20251112
gcc version	gcc (GCC) 15.2.1 20251112
clang version	clang version 21.1.6
tcc version	tcc version 0.9.28rc 2025-02-13 HEAD@f8bd136d (x86_64 Linux)
tcc git status	thirdparty-linux-amd64 696c1d84
emcc version	N/A
glibc version	ldd (GNU libc) 2.42

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