MiniMaven: be careful with .jar names when copying snapshot versions

When retrieving -SNAPSHOT versions from $HOME/.m2/, the source
file names can have timestamps and build numbers in place of the
'SNAPSHOT' substring. Typically this happens when using real Maven
(e.g. by using an IDE such as Eclipse). We need to make sure that
the target file name has the '-SNAPSHOT' suffix again.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>