Fix inspectTastyFilesInJar in TastyInspector

[romanowski]

Add integration test

fixes #12852

[nicolasstucki]

Now that we are re-compiling the full std-lib from the jar we hit another issue.

Test dotty.tools.dotc.BootstrappedStdLibTASYyTest.testFromTastyInJar failed: java.lang.AssertionError: assertion failed: class Unit, took 14.974 sec

It looks like scala/Unit.tasty is in the blacklist. Maybe there is a bug with the blacklisting logic or the filtering logic.

[BarkingBad]

The problem was the paths returned from traversing the Jar file were with prefix root indicator in its own filesystem (e. g. /scala/Unit.tasty) and we compared it to relative paths (e. g. scala/Unit.tasty). Removing the leading files separator should fix the blacklisting of the primitives and also should work with loading correct class names (the next transformation is replacing all the slashes / with the dots ., which in fact would create names with leading dot, e. g. .scala.collection.immutable.List

[nicolasstucki]

Otherwise LGTM