code: function isInstalled(dep) {
      if (_.has(exceptions, dep)) return exceptions[dep];
      return installedDeps.has(dep);
  }
docs: Check if the given dependency is installed.
  Checks against `dependencies`, `devDependencies` in project `package.json`.
  
  @param {string} dep Name of the dependency
  @returns {boolean} `true` if installed, `false` if not