fixes package loading for jars. only loads classes from the specified package

This PR fixes this issue: https://github.com/mongodb/morphia/issues/662

Given the following package structure:
com/foo/model/A.java
com/foo/model/bar/B.java

Calling ReflectionUtils.getClasses("com.foo.model") will load A.java when using the source code but both A.java and B.java when reading from a jar. This fix will only load A.java in both cases.