The original design for checkstyle and the Java mixin for tools provided

for a single configured classpath, defined as java_classpath in the

configuration file or as the $CLASSPATH environment variable.
In practice, this doesn't work too well. Both PMD and checkstyle used

some of the same dependencies, bundled in both their respective jar

files. Depending on the versions, these dependencies would conflict when

both were in a classpath.
Rather than having a single classpath, there's now tool-specific

classpaths, defined as tool keys in java_classpaths in the

configuration file.
The $CLASSPATH environment variable is no longer supported or

recommended, but if it's set for the process, it will still bubble

through. We may not want to support this long-term, but we get it for

free right now due to how execute() works, and it's not worth the

effort of preventing it.

All unit tests passed.