Avoid different chunks' content after each build for Vue components with @apply

[hivokas]

When we use @apply in the styles of the Vue component, the resulting chunks' content is different after each build (the reason why I want chunks' content to be the same after each build described here: #3349).

And the difference lays here:

That's because PostCSS generates random Input#id if from is not provided (https://postcss.org/api/#input-id).

So in order to fix the problem described above, from needs to be provided. That's what this PR does.

I was wondering about the best value for the from option and had decided to use __filename since the idea of the from option is to help users to find the source of the CSS (thanks @ai: postcss/postcss#1512 (comment))

Fixes #3349

[codecov-io]

Codecov Report

Merging #3356 (b23f8f8) into master (3fa7a22) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3356   +/-   ##
=======================================
  Coverage   93.28%   93.28%           
=======================================
  Files         178      178           
  Lines        1831     1831           
  Branches      324      323    -1     
=======================================
  Hits         1708     1708           
  Misses        105      105           
  Partials       18       18           

Impacted Files	Coverage Δ
src/lib/substituteClassApplyAtRules.js	95.12% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3fa7a22...b23f8f8. Read the comment docs.

[adamwathan]

Thank you, this looks solid 👍🏻 Appreciate you digging deep and contributing the solution to your problem, I'll try and get a tag out for this soon.

[hivokas]

Thank you, this looks solid 👍🏻 Appreciate you digging deep and contributing the solution to your problem, I'll try and get a tag out for this soon.

Thanks for merging!