[jvm] better error message when a test exists in a transparent package

`selfie.*` is transparent, so if a test is there, it fails like this:

```
org.opentest4j.AssertionFailedError: Couldn't find source file for CallLocation(clazz=org.junit.runners.model.FrameworkMethod$1, method=, fileName=FrameworkMethod.java, line=-1), looked in TypedPath(absolutePath=/home/runner/work/diffplugdotcom/diffplugdotcom/server/src/test/java/) and [/home/runner/work/diffplugdotcom/diffplugdotcom/server/src/test/resources], maybe there are other source roots?
at app//com.diffplug.selfie.junit5.FSJava.assertFailed(SnapshotSystemJUnit5.kt:57)
at app//com.diffplug.selfie.junit5.FSJava.assertFailed(SnapshotSystemJUnit5.kt:43)
at app//com.diffplug.selfie.guts.FS.assertFailed$default(SnapshotSystem.kt:96)
at app//com.diffplug.selfie.junit5.SnapshotFileLayoutJUnit5.sourcePathForCall(SnapshotFileLayoutJUnit5.kt:44)
at app//com.diffplug.selfie.guts.CommentTracker.hasWritableComment(CommentTracker.kt:36)
at app//com.diffplug.selfie.junit5.SnapshotSystemJUnit5.sourceFileHasWritableComment(SnapshotSystemJUnit5.kt:90)
at app//com.diffplug.selfie.Mode.canWrite$selfie_lib(Mode.kt:32)
at app//com.diffplug.selfie.SelfieImplementationsKt.checkSrc(SelfieImplementations.kt:285)
at app//com.diffplug.selfie.SelfieImplementationsKt.access$checkSrc(SelfieImplementations.kt:1)
at app//com.diffplug.selfie.StringSelfie.toBe(SelfieImplementations.kt:206)
at app//selfie.ParsedFormsTest.toMd(ParsedFormsTest.kt:16)
```

Reproduziere den Fehler aus ParsedFormsTest.kt und verfolge die Quelltextsuche durch SnapshotFileLayoutJUnit5.kt und SnapshotSystemJUnit5.kt. Bestimme, wie transparente Pakete behandelt werden, sorge dann dafür, dass der Fehler die fehlende Quelle klarer erklärt, und überprüfe die resultierende Meldung mit dem betroffenen Test.

Vom Indexierungsmodell aus dem Issue-Text verfasst.