split package usage: "org.gitlab4j.api.systemhooks" & "org.gitlab4j.api.webhook"

[jmini]

The separation into 2 different modules (see #1067) is not really clean yet:

Detected a split package usage which is considered a bad practice and should be avoided. Following packages were detected in multiple archives: 
- "org.gitlab4j.api.systemhooks" found in [org.gitlab4j:gitlab4j-api:6.0.0-rc.7, org.gitlab4j:gitlab4j-models:6.0.0-rc.7]
- "org.gitlab4j.api.webhook" found in [org.gitlab4j:gitlab4j-api:6.0.0-rc.7, org.gitlab4j:gitlab4j-models:6.0.0-rc.7]