use key "packages" for requirements instead of "addons"/"plugins"
simplified i18n error messages
use <b> for package messages
use composer-like version constraints
package conflicts
better doc comment
package manager: always return boolean in check* methods
separated checkRequirements and checkConflicts
simplified $messages
Merge pull request #194 from redaxo/package-requirements-and-conflicts

Conflicts-Angabe in package.yml / Änderung "requires"-Angabe