Add support for major release selection

This commit adds 'latest' version alias that resolves to the
latest available release.

The getPossibleVersions function filters available releases using
startsWith(version), so calling getLatestVersion(version) with an
empty string will return the latest release.

Closes #31

__tests__/installer.test.ts

@@ -89,6 +89,18 @@ describe('installer tests', () => {
         expect(fs.existsSync(path.join(goDir, 'bin', 'go'))).toBe(true);
       }
     }, 100000);
+
+    it('Acquires latest release version of go if using latest and matching version is installed', async () => {
+      await installer.getGo('latest');
+      const goDir = path.join(toolDir, 'go', '1.13.0', os.arch());
+
+      expect(fs.existsSync(`${goDir}.complete`)).toBe(true);
+      if (IS_WINDOWS) {
+        expect(fs.existsSync(path.join(goDir, 'bin', 'go.exe'))).toBe(true);
+      } else {
+        expect(fs.existsSync(path.join(goDir, 'bin', 'go'))).toBe(true);
+      }
+    }, 100000);
   });
 
   it('Throws if no location contains correct go version', async () => {

src/installer.ts

@@ -167,6 +167,10 @@ function normalizeVersion(version: string): string {
 }
 
 async function determineVersion(version: string): Promise<string> {
+  if (version == 'latest') {
+    return await getLatestVersion('');
+  }
+
   if (!version.endsWith('.x')) {
     const versionPart = version.split('.');
 
@@ -188,7 +192,7 @@ async function getLatestVersion(version: string): Promise<string> {
 
   core.debug(`evaluating ${versions.length} versions`);
 
-  if (version.length === 0) {
+  if (versions.length === 0) {
     throw new Error('unable to get latest version');
   }