De-duplicate hoisted imports and avoid rewriting imports of built-ins

[amannn]

Since division with / is being deprecated I stumbled upon two issues while moving to sass:math with sass-resources-loader.

This PR addresses both of them:

• Hoisted imports should be de-duplicated so SASS doesn't throw. I'd argue that this is necessary, since an individual style sheet can't know which imports are used in a resource and also shouldn't rely on them.
• Imports to built-ins from SASS shouldn't be rewritten.

I've tested the changes from this PR locally in my project and everything worked fine then.

Thank you for maintaining sass-resources-loader!

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This change is 

[amannn]

Previously this was ../sass:math

[amannn]

@justin808 Do you think you'd have time for a brief review here?

[justin808]

Hi @amannn, thanks for reaching out, and thanks for your contribution!

I took a glance, and it looks good.

How confident are you that:

1. Your change won't cause any regressions.
2. You added adequate tests.
3. You added adequate documentation.
4. You added a changelog entry. What should be the next version number?

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@Tomburgs, any chance that you can take a quick look

[CyberCookie]

Please merge!
There is no way to @use "sass:math" in sass files together with this lib since the lib adds resources to the very top of sass files and placing @use rule manually produces rules ordering error
sass use
"A stylesheet’s @use rules must come before any rules..."

[amannn]

Hi @justin808,

thanks for having a look!

I've added some new tests and the previous tests seem to still work, so I hope the change is ok. An in-depth look from you would of course be really helpful. There are no necessary docs changes, as this is only a compatibility fix. I've added a changelog entry and bumped the version number as requested.

Sorry, I closed the PR by accident before but reopened it now.

[justin808]

@CyberCookie @amannn 2.2.2 pushed to npm!