Cli: refactor: use async fs calls in src/utils/proxy.js

[JCMais]

Describe the bug

This file could easily use the async versions of the fs functions instead of the sync ones.

Additional context

See: https://github.com/netlify/cli/pull/1271#discussion_r493164135

[mugi111]

If you don't mind, I want to challenge to fix this issue !

[erezrokah]

Thanks @mugi111, I assigned you to the issue. Please let me know if there is anything you need.

[mugi111]

https://github.com/netlify/cli/blob/5c9fed77e3aaa632c154000180d692ca0742dd1a/src/utils/proxy.js#L235
Let me ask for advice on how to fix this.
Is it correct to filter the headersFiles with fs.stat() and isFiles() once and then use map() for that array?

[erezrokah]

You could use https://github.com/sindresorhus/p-filter to filter promises, then run map in the result.
You can use https://github.com/netlify/cli/blob/5c9fed77e3aaa632c154000180d692ca0742dd1a/src/lib/fs.js#L18 for the filter

[mugi111]

Thank you. It was helpful.
I'll deal with it that way.