Add go-version-file option (#62)

src/installer.ts

@@ -4,6 +4,7 @@ import * as path from 'path';
 import * as semver from 'semver';
 import * as httpm from '@actions/http-client';
 import * as sys from './system';
+import fs from 'fs';
 import os from 'os';
 
 type InstallationType = 'dist' | 'manifest';
@@ -298,3 +299,14 @@ export function makeSemver(version: string): string {
   }
   return fullVersion;
 }
+
+export function parseGoVersionFile(versionFilePath: string): string {
+  const contents = fs.readFileSync(versionFilePath).toString();
+
+  if (path.basename(versionFilePath) === 'go.mod') {
+    const match = contents.match(/^go (\d+(\.\d+)*)/m);
+    return match ? match[1] : '';
+  }
+
+  return contents.trim();
+}