_normalizePath (p) {
    if (path.isAbsolute(p)) {
      p = path.relative(this.generator.context, p)
    }
    // The `files` tree always use `/` in its index.
    // So we need to normalize the path string in case the user passes a Windows path.
    return p.replace(/\\/g, '/')
  }
Normalize absolute path, Windows-style path
to the relative path used as index in this.files
@param {string} p the path to normalize