"Many Maven plugins can get this resolution automatically if they declare their affected mojo parameters of type java.io.File instead of java.lang.String. This subtle difference in parameter types will trigger a feature known as path translation, i.e. the Maven core will automatically resolve relative paths when it pumps the XML configuration into a mojo."
When there are nested projects, target/mods was not resolving correctly.
See http://maven.apache.org/plugin-developers/common-bugs.html#Resolving_Relative_Paths for an explanation of how to resolve relative paths
"Many Maven plugins can get this resolution automatically if they declare their affected mojo parameters of type java.io.File instead of java.lang.String. This subtle difference in parameter types will trigger a feature known as path translation, i.e. the Maven core will automatically resolve relative paths when it pumps the XML configuration into a mojo."