Handle dynamic css imports that are also statically imported

[benmccann]

Fixes #1493. See the issue for a description of the change

[Conduitry]

Can we add a test for this as well? These were obviously weird edge cases that were never considered, and it would be nice to have some ability to guard against them going forward.

[benmccann]

Ok, I updated the tests so that they fail on master and pass with this PR. I didn't need to write a new test but just added a new route that would cause existing tests to fail

[Conduitry]

Oh nice, I see that this also makes rollup-dependency-tree a dev/bundled dependency.

[benmccann]

My reason for moving it was that it's only used in building the application by Rollup / the Sapper build process and so I was thinking it was a dev dependency. E.g. if someone runs npm ci --prod then I figured this would stop it from being a dependency that's shipped to production. It works just fine either place I have it and I don't have much of an opinion on where to put it. Were you thinking I should move it back?

[Conduitry]

No no, it's nice having it be bundled in instead. My understanding from the conversation here was that you were running into some issues when doing that, but it didn't seem very worthwhile to try to get to the bottom of it, because Sapper itself is only a dev dependency of apps that are written in it, and so built apps didn't have any additional dependencies.

I'm a bit confused by parts of your last comment. I would expect that the only difference for someone installing sapper into their app would be whether they get a bundled copy of rollup-dependency-tree or their package manager has to separately request and install it.

[benmccann]

I think we may have been talking about different things when you said "external" in #1452 (comment). I originally thought you meant was there a reason to make it a library and get it from npm instead of just putting the source code in Sapper. But now I understand you mean devDependencies vs not, so I was answering a bit of a different question on the other thread.

Feel free to ignore my last comment too 😄 I get a bit confused sometimes about Sapper being both a builder and a runtime in the same project

Anyway, I don't think there's much difference whether this is in devDependencies or dependencies, so I'm happy to have it in either place