For a given package, enumerate its internal dependencies (other packages in the repo) and symlink them into the appropriate spot in `node_modules`.
function symlinkDependencies(pkg) {
      let name = pkg.name;
      let modulesPath = path.join(outputPath, name, 'node_modules');
      let dependencies = pkg.internalDependencies;

      // @glimmer/test-helpers has some tricky circular dependencies so we manually
      // link it into every package locally.
      if (name !== '@glimmer/test-helpers') {
        dependencies.push('@glimmer/test-helpers');
      }

      dependencies.forEach(dep => {
        if (isScopedPackage(dep)) { mkdirpScope(dep, modulesPath); }
        fs.symlinkSync(
          path.join('../../../../', dep),
          path.join(modulesPath, dep)
        );
      });
    }