Can I use packages inside build.zig?

Brainstorming
6

Yes, your understanding is correct.

essentially I tried have build.zig.zon

.{
    .name = "build_use_package",
    // This is a [Semantic Version](https://semver.org/).
    // In a future version of Zig it will be used for package deduplication.
    .version = "0.0.0",

    // This field is optional.
    // This is currently advisory only; Zig does not yet do anything
    // with this value.
    //.minimum_zig_version = "0.11.0",

    // This field is optional.
    // Each dependency must either provide a `url` and `hash`, or a `path`.
    // `zig build --fetch` can be used to fetch all dependencies of a package, recursively.
    // Once all dependencies are fetched, `zig build` no longer requires
    // Internet connectivity.
    .dependencies = .{
        .zcmd = .{
            .url = "https://github.com/liyu1981/zcmd.zig/archive/refs/tags/v0.2.0.tar.gz",
            .hash = "1220bb5963c28e563ed010e5d622611ec0cb711ba8c6644ab44a22955957b1b8fe1a",
        },
    },
    .paths = .{
        // This makes *all* files, recursively, included in this package. It is generally
        // better to explicitly list the files and directories instead, to insure that
        // fetching from tarballs, file system paths, and version control all result
        // in the same contents hash.
        "",
        // For example...
        //"build.zig",
        //"build.zig.zon",
        //"src",
        //"LICENSE",
        //"README.md",
    },
}

and build.zig

const std = @import("std");
const zcmd = @import("zcmd");

pub fn build(b: *std.Build) !void {
    const target = b.standardTargetOptions(.{});
    const optimize = b.standardOptimizeOption(.{});

    const lib = b.addStaticLibrary(.{
        .name = "build_use_package",
        .root_source_file = .{ .path = "src/root.zig" },
        .target = target,
        .optimize = optimize,
    });

    b.installArtifact(lib);

    const exe = b.addExecutable(.{
        .name = "build_use_package",
        .root_source_file = .{ .path = "src/main.zig" },
        .target = target,
        .optimize = optimize,
    });

    // do something with zcmd like
    const result = try zcmd.run(.{
        .allocator = std.heap.page_allocator,
        .commands = &[_][]const []const u8{
            &.{ "uname", "-a" },
        },
    });
    defer result.deinit();
    // next do something with result.stdout like extract part of the OS's info

    b.installArtifact(exe);
    const run_cmd = b.addRunArtifact(exe);
    run_cmd.step.dependOn(b.getInstallStep());
    if (b.args) |args| {
        run_cmd.addArgs(args);
    }
    const run_step = b.step("run", "Run the app");
    run_step.dependOn(&run_cmd.step);

    const lib_unit_tests = b.addTest(.{
        .root_source_file = .{ .path = "src/root.zig" },
        .target = target,
        .optimize = optimize,
    });

    const run_lib_unit_tests = b.addRunArtifact(lib_unit_tests);

    const exe_unit_tests = b.addTest(.{
        .root_source_file = .{ .path = "src/main.zig" },
        .target = target,
        .optimize = optimize,
    });

    const run_exe_unit_tests = b.addRunArtifact(exe_unit_tests);

    const test_step = b.step("test", "Run unit tests");
    test_step.dependOn(&run_lib_unit_tests.step);
    test_step.dependOn(&run_exe_unit_tests.step);
}

when zig build, it will not work, which is understandable, as zig runtime itself is not knowing where zcmd is (as no b.depency and exe.addModule step)

so I am currently add my mod as submodules in local, but I am wishing the url and hash way can also be supported somehow in zig