Incremental fixes, refactor Zcu.File

[mlugg]

See commit messages.

There is one breaking change here, which is that @import("non_existent_module") is now a compile error even if the import is never analyzed. This is going to fail CI at the moment, because Aro currently relies on the old behavior of accepting these imports.

[NicoElbers]

In one of my project I currently use non existant modules to have optional depencencies. Simplified slightly below:

// build.zig
const use_jemalloc = b.option(bool, "use_jemalloc", "test jemalloc") orelse false;

const exe_opts = b.addOptions();
exe_opts.addOption(bool, "use_jemalloc", use_jemalloc);

const exe_mod = b.createModule(.{
    // ...
});

exe_mod.addOptions("build_opts", exe_opts);

// the jemalloc source code may not be downloaded, or maybe exe_mod wants to stay libc free
if (use_jemalloc) {
    const jemalloc_mod = b.createModule(.{
        // ...
    });

    exe_mod.addImport("use_jemalloc", jemalloc_mod);
}

// main.zig
const functions = [_]Contructor{} ++
    if (build_opts.use_jemalloc) @import("jemalloc").constructor else .{};

const build_opts = @import("build_opts");

// ...

If I understand correctly, after this PR this code will no longer compile when use_jemalloc is false, as jemalloc_mod is never created. Is there a proper way to do optional dependencies? or am I doing something too weird.

[mlugg]

In that case, I'd suggest just adding the module in the build script regardless of use_jemalloc. It doesn't do any harm; the build option will still stop it from actually being analyzed.

There could be other cases I've not thought of where this is more of a problem, though. If anyone has any, feel free to ask here, and we can see if this language change is problematic.

[NicoElbers]

It may be useful to label this PR as breaking, so more people are inclined to check it out.