chore: fix export issue generate by webpack upgrading

[island205]

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Issue 1:

// Loading is undefined
import {Loading} form 'element-ui'

Issue 2:

like #14216

<script src="//unpkg.com/vue"></script>
<script src="//unpkg.com/element-ui"></script>
<script src="//unpkg.com/element-ui/lib/umd/locale/en.js"></script>

<script>
  // ELEMENT.locale is undefined, ELEMENT.default.locale is available
  ELEMENT.locale(ELEMENT.lang.en)
</script>

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