Tidy up structure of rules_jvm_external

[shs96c]

This is a no-op from a user's point of view, but it makes some changes
to the internals of rules_jvm_external, notably:

• defs.bzl now just re-exports definitions. Nothing is defined in it
• There is a build file in //private so that we don't need to remember where build files are
• All rules are now in their own *.bzl files