Fixes for CSS transformer

[devongovett]

Fixes #7858. Fixes #7859. Fixes #7865. Fixes #5985.

• Ensures we don't remove unused symbols in the CSS optimizer after the packager already did that (for CSS modules processed by postcss)
• Only adds dependencies to the JS asset of a CSS module, rather than both the JS and CSS. This avoids an issue where a dependency in CSS resolves to a JS asset, which breaks bundling. In order to make this work, I also needed to solve an issue in both bundlers, where dependencies in the JS part of a CSS module were included after the sibling CSS rather than before. This is solved by queuing assets to be added to sibling CSS bundles during post-order traversal rather than adding them during pre-order.
• Sets the environment before adding dependencies rather than after. Replacing the environment also affects an asset's id, which is used in dependencies. If they don't match this breaks things downstream in the bundle graph when transforming for multiple environments, causing errors like "Error: Does not have node" when importing assets and using module/nomodule pattern #5985.
• Uses postcss-modules to handle :export syntax in legacy CSS modules.

[parcel-benchmark]

Benchmark Results

Kitchen Sink 🚨

Timings

Description	Time	Difference
Cold	FAILED	-0.00ms
Cached	FAILED	-0.00ms

Cold Bundles

No bundles found, this is probably a failed build...

Cached Bundles

No bundles found, this is probably a failed build...

React HackerNews ✅

Timings

Description	Time	Difference
Cold	12.23s	+82.00ms
Cached	543.00ms	-2.00ms

Cold Bundles

No bundle changes detected.

Cached Bundles

No bundle changes detected.

AtlasKit Editor 🚨

Timings

Description	Time	Difference
Cold	FAILED	-0.00ms
Cached	FAILED	-0.00ms

Cold Bundles

No bundles found, this is probably a failed build...

Cached Bundles

No bundles found, this is probably a failed build...

Three.js ✅

Timings

Description	Time	Difference
Cold	8.65s	+31.00ms
Cached	337.00ms	-7.00ms

Cold Bundles

No bundle changes detected.

Cached Bundles

No bundle changes detected.

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[mischnic]

The first issue you mentioned is probably the wrong one?

[mischnic]

Looks good, apart from the bundlers - I have absolutely no clue how they work at this point.

If they don't match this breaks things downstream in the bundle graph when transforming for multiple environments, causing errors like

Ideally we'd somehow prevent this from happening in the first place. And if that's not possible, maybe throw an error if you do setEnvironment before addDependency so that this doesn't go unnoticed.

[devongovett]

Ideally we'd somehow prevent this from happening in the first place

Yeah... setEnvironment is really a hack. Ideally we'd replace it with something better, but not sure what.

[aminomancer]

Are you sure #7858 was fixed? I'm still experiencing the same issue on 2.8.3