copyPluginDescriptorInputs should be out-of-date when the project's version changes

[wilkinsona]

copyPluginDescriptorInputs copies a pom.xml file that used to generate the Maven plugin's plugin descriptor. During the copy it replaces a placeholder in the pom file with the project's current version. However, the version isn't recorded as an input of the task so when it changes the task may still be considered up-to-date. This results in the descriptor being generated with the wrong version number.