feat(go): version file in the generated module directory (#2492)

Since golang version isn't available anywhere inside the module, we embed a version file so that consumers can access the version. 

This is needed for publishing [go modules](cdklabs/publib#23) using `jsii-release` without the user having to supply the version externally.

---

By submitting this pull request, I confirm that my contribution is made under the terms of the [Apache 2.0 license].

[Apache 2.0 license]: https://www.apache.org/licenses/LICENSE-2.0

packages/jsii-pacmak/lib/targets/go/package.ts

@@ -14,6 +14,7 @@ import {
 } from './runtime';
 import { GoClass, GoType, Enum, Interface, Struct } from './types';
 import { findTypeInTree, goPackageName, flatMap } from './util';
+import { VersionFile } from './version-file';
 
 export const GOMOD_FILENAME = 'go.mod';
 export const GO_VERSION = '1.15';
@@ -184,6 +185,7 @@ export class RootPackage extends Package {
   public readonly assembly: Assembly;
   public readonly version: string;
   private readonly readme?: ReadmeFile;
+  private readonly versionFile: VersionFile;
 
   public constructor(assembly: Assembly) {
     const packageName = goPackageName(assembly.name);
@@ -201,6 +203,7 @@ export class RootPackage extends Package {
 
     this.assembly = assembly;
     this.version = assembly.version;
+    this.versionFile = new VersionFile(this.version);
 
     if (this.assembly.readme?.markdown) {
       this.readme = new ReadmeFile(
@@ -216,6 +219,7 @@ export class RootPackage extends Package {
     this.readme?.emit(context);
 
     this.emitGomod(context.code);
+    this.versionFile.emit(context.code);
   }
 
   private emitGomod(code: CodeMaker) {

packages/jsii-pacmak/lib/targets/go/version-file.ts

@@ -0,0 +1,17 @@
+import { CodeMaker } from 'codemaker';
+
+/**
+ * Represents the version of the go module. This is needed because the version is not
+ * available in standard go module files.
+ */
+export class VersionFile {
+  private static readonly NAME = 'version';
+
+  public constructor(private readonly version: string) {}
+
+  public emit(code: CodeMaker) {
+    code.openFile(VersionFile.NAME);
+    code.line(this.version);
+    code.closeFile(VersionFile.NAME);
+  }
+}