Adjust forge.maven plug-in to 4.3 API change.
(GEP-113) Add deprecation warning for use of import.

This commit adds a new 'ImportIsDeprecated' validation issue and
code that will report when the import keyword is used. The severity
can be controlled by a validation preference which is set to 'ignore'
for target platforms below 3.5 and to 'warning' for 3.5 and above.