Issue: #2005

Prior to this commit, all JAR files generated for a published module
included their compiled module descriptor within the root directory
of the artifact.

This commit a) includes the compiled module descriptor only in JAR
files that have no classifier specified and b) stores the source of
each module descriptor within the matching `*-source.jar` file.

Fixes #2003

Prior to this commit, the JupiterTestEngine would crash without
executing any tests if JUnit 4 was on the classpath but Hamcrest was
not.

This commit fixes this bug by ensuring that initialization of the
OpenTest4JAndJUnit4AwareThrowableCollector class no longer fails if the
org.junit.internal.AssumptionViolatedException class cannot be loaded
from the classpath -- for example, due to a missing Hamcrest dependency.

Fixes #2004

Issue: #2005

Issue #2004

This commit improves diagnostics by logging the cause of a failure to
load the AssumptionViolatedException class in
OpenTest4JAndJUnit4AwareThrowableCollector.

Issue: #2004