fix: respect rollupOptions.external for transitive dependencies

[haoqunjiang]

Description

The regression was brought in by #8280

To be compatible with Vite 2.x behavior:
When rollupOptions.external is set to ['vue'],

• import ... from 'vue' in dependencies should be preserved;
• const Vue = require('vue') in dependencies should be transformed to import * as Vue from 'vue' and be preserved;

Ideally, we need a first-class build.external option, but that belongs to another issue/PR. It's fine to postpone it after 3.0 stable is out.

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Remaining issues in this PR:

• rollupOptions.external only accepts string[] | string in the current implementation
  • this is because optimizeDeps.exclude only supports string[]
  • Rollup supports RegExp | RegExp[] and function formats https://rollupjs.org/guide/en/#external
  • If we want to support all these types, we need to extend the ability of optimizeDeps.exclude, either exposing it internally, or open to all end users.
  • Do we really want to do that? Or we can do that in 3.1 or later…
• The CJS external implementation is naive, there could potentially be more edge cases than in v2. But I think it's acceptable, because this use case shouldn't be encouraged at all.

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
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[patak-dev]

Awesome! Looks good to me, merging so we can include it in beta.1