incompatible_java_coverage: allow coverage collection for JVM languages
The option `--incompatible_java_coverage`:

- makes possible collecting coverage for Starlark JVM languages and jars imported with `java_import`
- only includes JVM files in the coverage report (dismisses data files, for example `.js` files that shouldn't have been in the report in the first place)
- the report includes the actual path of the files relative to the workspace root instead of the package path (e.g. `src/com/google/Action.java` instead of `com/google/Action.java`). This allows `genhtml` to generate HTML reports from the combined bazel coverage output.

For more details see [Bazel coverage for JVM languages](https://docs.google.com/document/d/1mQLQa2uMIgVcwTE-hwaKgwQ5upExIRH5p9lHFyxhj4g/edit?usp=sharing).

# Migration Notes

The users do not have to do any migration. Having the package paths in the report was filed as a bug (see #2528).

src/main/java/com/google/devtools/build/lib/analysis/config/BuildConfiguration.java
src/main/java/com/google/devtools/build/lib/analysis/config/BuildConfiguration.java
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/config/BuildConfiguration.java b/src/main/java/com/google/devtools/build/lib/analysis/config/BuildConfiguration.java
index 4347d97681d619..0732a81409504f 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/config/BuildConfiguration.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/config/BuildConfiguration.java
@@ -589,7 +589,7 @@ public static class Options extends FragmentOptions implements Cloneable {
 
     @Option(
         name = "incompatible_java_coverage",
-        defaultValue = "false",
+        defaultValue = "true",
         documentationCategory = OptionDocumentationCategory.OUTPUT_PARAMETERS,
         effectTags = {OptionEffectTag.AFFECTS_OUTPUTS},
         metadataTags = {
