Properly identify and check Scala.js dependencies

[keynmol]

Closes #27

It felt like almost luck to discover that sbt-platform-deps puts stuff into the Binary :D

@cb372 I'm putting this PR up in relatively incomplete state for the following reasons:

• Verify Scala.js 0.6.x prefix/suffix
• Check that it's not important to verify full Cross versions for platforms

But more importantly: to make sure you agree with the approach.

I would like to do the same changes for Scala Native dependencies, but given the relative usage stats it can be a separate PR/release.

[keynmol]

@cb372 👋 this PR is 2 years old so I don't quite remember what I was doing - but if you are interested in this functionality, I'll be available to address feedback!

[cb372]

So you noticed that this plugin has risen from the dead!

Thank you for this PR, which I totally missed the first time around. A couple of quick comments before I review properly:

1. I got rid of the sbt-test/1.3+ folder in Test against a more recent version of sbt #128 so all the tests are now in sbt-test/basic. Could you move your tests there please.
2. Do we still care about supporting Scala.js 0.6.x these days? I'm not that familiar with the ecosystem, so it's a genuine question.