fix: dependency processing race condition

[tivac]

Description

Ensure that walking the tree has completed before allowing exports/dependency information to be flattened.

Motivation and Context

If you called processor.string() or processor.file() on two files with shared dependencies w/o waiting for the first file to complete it's walk of the tree you could get random errors that made no sense.

How Has This Been Tested?

Added a svelte & non-svelte test for it.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #565 into master will decrease coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #565      +/-   ##
==========================================
- Coverage   99.06%   98.97%   -0.09%     
==========================================
  Files          49       49              
  Lines        1173     1176       +3     
  Branches      183      183              
==========================================
+ Hits         1162     1164       +2     
- Misses         11       12       +1

Impacted Files	Coverage Δ
packages/processor/processor.js	100% <100%> (ø)	⬆️
packages/svelte/svelte.js	100% <100%> (ø)	⬆️
packages/webpack/loader.js	91.66% <0%> (-4.17%)	⬇️

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[deflock]

I'm not sure if this is related, but sometimes I get Error: Use process(css).then(cb) to work with async plugins when using postcss-import in before.