Currently MojoAnnotationsScanner always scans all dependencies in search for Mojo's. However, most of the time there's no need to do so: the sources are all the mojo's for the plugin.
The simple solution would be to specify if the plugin should scan, and maybe even which dependencies.
A more elegant way would be to analyze the source-classes. If the Mojo's extend known classes like AbstractMojo, there's no need to scan at all.
ps. plugins which require dependencies-scanning are the maven-surefire-plugin and maven-failsafe-plugin