Use join instead of resolve and update path import

This fixes `pkg` bundling

Author: bradlc

src/corePlugins.js

@@ -1,5 +1,5 @@
 import fs from 'fs'
-import path from 'path'
+import * as path from 'path'
 import postcss from 'postcss'
 import createUtilityPlugin from './util/createUtilityPlugin'
 import buildMediaQuery from './util/buildMediaQuery'
@@ -365,7 +365,7 @@ export let variantPlugins = {
 export let corePlugins = {
   preflight: ({ addBase }) => {
     let preflightStyles = postcss.parse(
-      fs.readFileSync(path.resolve(__dirname, 'css/preflight.css'), 'utf8')
+      fs.readFileSync(path.join(__dirname, './css/preflight.css'), 'utf8')
     )
 
     addBase([