Fix "Unused imported module" rule

[ljmf00]

• core.runtime is reported unused but Runtime object is being used.
• std is reported unused when import core.runtime; is present.

---

import std;
import core.runtime;

UnitTestResult customModuleUnitTester()
{
    return UnitTestResult(100, 100, false, true);
}

shared static this()
{
    Runtime.extendedModuleUnitTester = &customModuleUnitTester;
}

void main()
{
    import core.stdc.stdio;
    fprintf(stderr, "main\n");
    writeln("f");
}