chore: generate go.mod files

Closes aws#2090

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

@@ -14,6 +14,9 @@ import {
 import { GoClass, GoType, Enum, Interface, Struct } from './types';
 import { findTypeInTree, goPackageName, flatMap } from './util';
 
+export const GOMOD_FILENAME = 'go.mod';
+export const GO_VERSION = '1.15';
+
 /*
  * Package represents a single `.go` source file within a package. This can be the root package file or a submodule
  */
@@ -66,18 +69,22 @@ export abstract class Package {
   }
 
   /*
-   * The module names of this modules dependencies. Used for import statements
+   * goModuleName returns the full path to the module name.
+   * Used for import statements and go.mod generation
+   */
+  public get goModuleName(): string {
+    const moduleName = this.root.moduleName;
+    const prefix = moduleName !== '' ? `${moduleName}/` : '';
+    const rootPackageName = this.root.packageName;
+    const suffix = this.filePath !== '' ? `/${this.filePath}` : ``;
+    return `${prefix}${rootPackageName}${suffix}`;
+  }
+
+  /*
+   * The module names of this module's dependencies. Used for import statements.
    */
   public get dependencyImports(): Set<string> {
-    return new Set(
-      this.dependencies.map((pack) => {
-        const moduleName = pack.root.moduleName;
-        const prefix = moduleName !== '' ? `${moduleName}/` : '';
-        const rootPackageName = pack.root.packageName;
-        const suffix = pack.filePath !== '' ? `/${pack.filePath}` : ``;
-        return `${prefix}${rootPackageName}${suffix}`;
-      }),
-    );
+    return new Set(this.dependencies.map((pkg) => pkg.goModuleName));
   }
 
   /*
@@ -89,6 +96,7 @@ export abstract class Package {
 
   public emit(context: EmitContext): void {
     const { code } = context;
+
     code.openFile(this.file);
     this.emitHeader(code);
     this.emitImports(code);
@@ -189,8 +197,25 @@ export class RootPackage extends Package {
     super.emit(context);
     this.emitJsiiPackage(context);
     this.readme?.emit(context);
+
+    this.emitGomod(context.code);
   }
 
+  private emitGomod(code: CodeMaker) {
+    code.openFile(GOMOD_FILENAME);
+    code.line(`module ${this.goModuleName}`);
+    code.line();
+    code.line(`go ${GO_VERSION}`);
+    code.line();
+    code.open('require (');
+    for (const packageName of this.dependencyImports) {
+      code.line(`${packageName}`);
+    }
+    code.close(')');
+    code.closeFile(GOMOD_FILENAME);
+  }
+
+
   /*
    * Override package findType for root Package.
    *