init template: comment and migrate from deprecated std.Build API

Restructurized "build.zig" and added more info about modules,
since they are now exposed here.

Signed-off-by: Eric Joldasov <bratishkaerik@landless-city.net>

lib/init/build.zig

@@ -15,25 +15,59 @@ pub fn build(b: *std.Build) void {
     // set a preferred release mode, allowing the user to decide how to optimize.
     const optimize = b.standardOptimizeOption(.{});
 
-    const lib = b.addStaticLibrary(.{
-        .name = "$",
+    // Here we are creating a module. Module is a set of files
+    // that you can use to build executable or library, or import into other module.
+    //
+    // Here we are creating a module, but not exposing it to package manager.
+    // To expose it to package manager, use function `addModule` and choose
+    // a name for it. Packages that want to import your module will need to use
+    // exactly this name in their build.zig:
+    //
+    // ```zig
+    // // In your build.zig
+    // const main_mod = b.addModule("main", .{ ... });
+    //
+    // // In their build.zig
+    // const some_dep = b.dependency("...", .{ ... });
+    // const some_mod = some_dep.module("main");
+    // ```
+    const main_mod = b.createModule(.{
         // In this case the main source file is merely a path, however, in more
         // complicated build scripts, this could be a generated file.
-        .root_source_file = b.path("src/root.zig"),
+        .root_source_file = b.path("src/main.zig"),
         .target = target,
         .optimize = optimize,
     });
 
-    // This declares intent for the library to be installed into the standard
-    // location when the user invokes the "install" step (the default step when
-    // running `zig build`).
-    b.installArtifact(lib);
+    // You can import to this module other Zig modules, add C/C++ source files, link libraries,
+    // and etc. To import Zig modules, use `addImport` function:
+    //
+    // ```zig
+    // main_mod.addImport("some_mod", some_mod);
+    // ```
+    //
+    // If they are added unconditionally, you can also import them during module creation instead:
+    // ```zig
+    // const main_mod = b.createModule(.{
+    //     // ...
+    //     .imports = &.{
+    //         .{ .name = "some_mod", .module = some_mod },
+    //         .{ .name = "another_mod", .module = another_mod },
+    //     },
+    // });
+    // ```
+    //
+    // Now you can leverage it in your source code by using `@import("some_mod")`
+    // syntax (get all definitions from root source file) or by using `@embedFile("some_mod")`
+    // (get literal content of the root source file at comptime).
+    //
+    // Note that Zig's "std" module (a.k.a. standard library)
+    // is always available, you don't need to import it manually.
 
-    const exe = b.addExecutable(.{
+    // After finishing module, we are building executable out of it.
+    const exe = b.addExecutable2(.{
         .name = "$",
-        .root_source_file = b.path("src/main.zig"),
-        .target = target,
-        .optimize = optimize,
+        .root_module = main_mod,
     });
 
     // This declares intent for the executable to be installed into the
@@ -66,26 +100,38 @@ pub fn build(b: *std.Build) void {
 
     // Creates a step for unit testing. This only builds the test executable
     // but does not run it.
-    const lib_unit_tests = b.addTest(.{
+    const exe_unit_tests = b.addTest2(.{
+        .root_module = main_mod,
+    });
+
+    const run_exe_unit_tests = b.addRunArtifact(exe_unit_tests);
+
+    // Similar to creating the run step earlier, this exposes a `test` step to
+    // the `zig build --help` menu, providing a way for the user to request
+    // running the unit tests.
+    const test_step = b.step("test", "Run unit tests");
+    test_step.dependOn(&run_exe_unit_tests.step);
+
+    // Below we are doing same things, but for static library.
+    const another_mod = b.createModule(.{
         .root_source_file = b.path("src/root.zig"),
         .target = target,
         .optimize = optimize,
     });
 
-    const run_lib_unit_tests = b.addRunArtifact(lib_unit_tests);
+    const lib = b.addLibrary(.{
+        .name = "$",
+        .root_module = another_mod,
+        .linkage = .static,
+    });
 
-    const exe_unit_tests = b.addTest(.{
-        .root_source_file = b.path("src/main.zig"),
-        .target = target,
-        .optimize = optimize,
+    b.installArtifact(lib);
+
+    const lib_unit_tests = b.addTest2(.{
+        .root_module = another_mod,
     });
 
-    const run_exe_unit_tests = b.addRunArtifact(exe_unit_tests);
+    const run_lib_unit_tests = b.addRunArtifact(lib_unit_tests);
 
-    // Similar to creating the run step earlier, this exposes a `test` step to
-    // the `zig build --help` menu, providing a way for the user to request
-    // running the 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);
 }