build: enable TSLint on the packages folder (#18459)

porting PRs #18392 and #18441 to 4.x

tools/gulp-tasks/lint.js

@@ -9,9 +9,13 @@ module.exports = (gulp) => () => {
         // todo(vicb): add .js files when supported
         // see https://github.com/palantir/tslint/pull/1515
         './modules/**/*.ts',
+        './packages/**/*.ts',
         './tools/**/*.ts',
         './*.ts',
 
+        // Ignore node_modules directories
+        '!**/node_modules/**',
+
         // Ignore TypeScript mocks because it's not managed by us
         '!./tools/@angular/tsc-wrapped/test/typescript.mocks.ts',