fix(v2): ignore import declarations in excerpt

[lex111]

Motivation

Resolve #2373
Resolve #2601

Have you read the Contributing Guidelines on pull requests?

Yes.

Test Plan

Compare MDX page with imports, eg:

Before	After
Page: https://deploy-preview-2380--docusaurus-2.netlify.com/docs/migrating-from-v1-to-v2 Description: import ColorGenerator from '@site/src/components/ColorGenerator';	Page: https://v2.docusaurus.io/docs/migrating-from-v1-to-v2/ Description: This doc guides you through migrating an existing Docusaurus 1 site to Docusaurus 2.

Related PRs

(If this PR adds or changes functionality, please take some time to update the docs at https://github.com/facebook/docusaurus, and link to your PR here.)

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[yangshun]

Import statements can be written anywhere on the page, so if users want to avoid this problem in most cases they can shift it below a little. I rather not add brittle hacks to our code.

[lex111]

But people usually import components at the very beginning of the file, so we are only interested in this place. And given the current architecture, this is a not bad solution.
I also do not consider this patch something like hack, since we need to prepare a description from the MD markup, and accordingly we need to pre-process it. Just the hack is that users will have to move import declarations to another place in the file to get rid of this bug.

[lex111]

Should I close this PR?

[yangshun]

Ok let's close this. Sorry about this.

[lex111]

No problem, although I did not understand why this is a bad way, given that we need to remove the imports declarations at the top of the file, in the same way we could remove Markdown markup.

[lex111]

Reopen after discussion in #2601

@yangshun what other use cases need to be considered?

[yangshun]

Thanks. Should we filter out require() statements as well? Some people might be using it.