Fix: @import url() error in dependency resolver

[groenroos]

What: Fixes #2595

Why: Because @import url() returns an error for the .deps() / --deps flag

How: Add a check to DepsResolver.prototype.visitImport to skip url() imports from dependency resolver, and thus from dependency lists

Checklist:

• Documentation
• Unit Tests
• Code complete

Comments

• It may be argued whether url() should or should not be included in the deps list (the documentation is not explicit about this) - however I've opted to skip them from the deps list, since they'll be almost necessarily external to the consumer codebase
• Replaces fix: DepsResolver.visitImport path isNull check #2627, as that solution causes other bugs in unit tests

[iChenLei]

https://developer.mozilla.org/en-US/docs/Web/CSS/@import#examples

Awesome pull request, thank you. I have a small request, can you add more complex unit test for this bugfix ? Current test case looks like function spec unit test, we can add more e2e-like tests, for example:

Input:

// import.url.styl
@import url('https://fonts.googleapis.com/css)
@import xxx;

// more code

Output:

// import.url.css
@import url('https://fonts.googleapis.com/css)

// gen code

There are many e2e-like tests in stylus/test/cases/*, more unit test cases is always a good thing, thank you again.

[groenroos]

@iChenLei Thanks for the review!

I reckoned the e2e tests were already covered by import.include.resolver.absolute.styl:

stylus/test/cases/import.include.resolver.absolute.styl

Lines 1 to 4 in 9cb7635

	@import url('http://foo.com/foo.css')
	@import url("http://foo.com/foo.css")
	@import url('https://foo.com/foo.css')
	@import url('//foo.com/foo.css')

In any case, this issue seems to be specific to generating the deps list with the --deps flag or the .deps() method - this issue seemed to never affect just parsing Stylus into CSS itself.

Let me know if there's some specific case that I should cover!

[groenroos]

@iChenLei Let me know if there is anything further to be done for this PR! I don't have write access so I cannot merge it myself.

[iChenLei]

@groenroos Check your permission please, I think you can merge this pr by yourself now. If encount any issue please @ me again. Thanks! Anyway, every pull request need peer review ! We should ensure pull request's quality.