fixed maven plugin classpath digester cache implementation

classpath digester ignored cached entry digests. in single-threaded
builds, first and second invocation of digester for the same plugin
always returned different digest, second and subsequent invocations
always returned same 'empty' digest. in multithreaded build,
digester returned different values first N invocations but
eventually settled on the same 'empy' value.

Signed-off-by: Igor Fedorenko <igor@ifedorenko.com>
moved all maven-related tests to the same package

Signed-off-by: Igor Fedorenko <igor@ifedorenko.com>