Support multi-project setups in the IDE tests

[Duhemm]

Because the IDE doesn't work with multi-project setup, I haven't included any test, but here's an example:

  @Test def classDefinition1: Unit = {
    val w0 = Workspace.withSources(
      code"""class ${m1}Foo${m2}""",
    )

    val w1 = Workspace.dependingOn(w0).withSources(
      code"""class Bar { new ${m3}Foo${m4} }"""
    )

    withWorkspaces(w0, w1)
      .definition(m1 to m2, List(m1 to m2))
      .definition(m3 to m4, List(m1 to m2))
  }

[smarter]

This is awesome! I would just rename workspace to project

[smarter]

Note for later: now that we have both from-source tests and from-tasty tests what would be really cool is to automatically check that we get the same results when loading from sources and when loading from tasty, in fact we could try to do this for all tests automatically: run them once with everything loaded from sources and once with everything loaded from tasty.