java-utils-2.eclass: Added new function java-pkg_gen-cp

java-utils-2.eclass: Added new function java-pkg_gen-cp

Java package generate classpath will create a classpath based on special
variable CP_DEPEND in the ebuild. This allows for automatic classpath
creation based on depends. Reduces chance of error in ebuild with slots
in deps not matching slots in classpath variable, etc. Not to most
elegant. Elected for multiple passes using bash vs external sed or awk.

Closes: https://github.com/gentoo/gentoo/pull/2286
Signed-off-by: James Le Cuirot <chewi@gentoo.org>