code: function queryImport(styles) {
      if (styles.indexOf('@import') < 0) return []
      const arr = []
      const ast = csstree.parse(styles);
      csstree.walk(ast, {
          enter: function (node) {
              if (node.type === 'Atrule' && node.name === 'import') {
                  const pat = node.prelude.children.head.data.value
                  arr.push(pat.substring(1, pat.length - 1))
              }
          }
      })
      return arr
  }
docs: get import content in css file,may import multi times
  @param styles css file content
  @returns {Array} result