Without the leading slash, the resulting Path ends up not having a
parent Path / directory. This in turn makes the (upcoming) backwards
compatibility adapter for the Linker fail.

While `Path`s without parents are techincally allowed, its hardly a
case we should care about for the backwards compatibility adapter.

Adding -Xexperimental makes compilation fail if the unapply is not
marked final: "could not optimize @tailrec annotated method unapply:
it is neither private nor final so can be overridden"

This commit does not add support to the compiler nor the sbt-plugin to
test backwards compatibility.

Further, we do not deprecate the old API yet to avoid build failures.

We cannot test this fully yet, since we need support in the sbt-plugin
for that.

At this point, we can fully test exports end-to-end. We do not migrate
all sbt-plugin tests to test backwards compatibility. However, we can
now deprecate the old APIs without build failures.

The custom-linker sbt test is migrated in this commit because it
relies on the internal scalaJSLinkerBox key.