Using the tokenVocab option in a grammar to reference a .tokens file that has ended up in a 'package' folder unfortunately requires you to specify the location of the generated file.
If, for example, you want to import a token vocabulary from a lexer in package my.pkg , just put the following in the mojo configuration:
...
<configuration>
<libDirectory>target/generated-sources/antlr/my/pkg</libDirectory>
</configuration>
...
In a future release, the plugin may be able to locate the file for itself, if within the same project, making this option redundant.