ability to `@import` dependencies directly in build.zig scripts

[andrewrk]

Extracted from #14265.

Within a build.zig file, @import should be allowed, and it should refer to the build.zig file of any dependencies listed in the manifest file.

Example:

build.zig.zon

    .dependencies = .{
        .@"android-sdk" = .{
            .url = "...",
            .hash = "...",
        },
    },

build.zig

const std = @import("std");

pub fn build(b: *std.build.Builder) void {
    const exe = b.addExecutable("app", "src/main.zig");

    @import("android-sdk").helperFunction(exe);
}

build.zig of android-sdk:

const std = @import("std");

pub fn build(b: *std.build.Builder) void {
    // ...
}

pub fn helperFunction(artifact: *std.build.LibExeObjStep) void {
    // ...
}

cc @MasterQ32

[ikskuh]

Yep that should be fully sufficient, as long as transitive dependencies work and will be properly deduplicated. So the following thing should work (assuming matching versions):

root_project
  -> library_a
  -> library_b
    -> library_c
      -> library_a

When library_b returns a type from library_a, those should be the same as the ones returned from the root_project dependency library_a.

[marler8997]

I implemented the ability to add recursive packages to build.zig in this PR: #8072

Also in that PR was an implementation for a proposal to add a buildpkgs builtin that build.zig could use to query optional dependencies.

That PR allowed you to do something like this:

const buildpkgs = @import("buildpkgs");
if (comptime buildpkgs.has("android-sdk")) {
    const android_sdk = @import("android-sdk");
    // code that handles having android-sdk
} else {
    // code that handles NOT having android-sdk
}

Looking back now it feels like there might be a slightly better interface. buildpkgs.zig could maybe look something more like this:

// if android sdk is available
pub const android_sdk: ?type = @import("android-sdk");
// if android sdk is not available
pub const android_sdk: ?type = null;

Then in build.zig

const buildpkgs = @import("buildpkgs");
if (buildpkgs.android_sdk) |android_sdk| {
    // code that handles having android-sdk
} else {
    // code that handles NOT having android-sdk
}

Not familiar with the new package manager constructs yet so the same concept might look different.