Compile-time constant dependency tracking moved to separate test.

The `final` modifier is not essential in `source-dependencies/export-jars`
and removing it eliminates the danger of running into Scala's bug SI-7173
where typer inlines constants and does not record any information about
dependency. In case we want to switch to dependency tracking based on
trees we would run into this problem in this test-case.

Testing for that scenario has been moved to separate test case
`source-dependencies/constants`.