Fix the pom: no module metadata, fix name and description

[simonbasle]

Since the switch to more recent version of Gradle, two issues have been introduced in the pom:

1. the name is wrong (doesn't pick up project.description from each submodule) - should be fixed by deferring the evaluation of project.description using afterEvaluate closure
2. the junit-platform submodule publishes module file, while agent doesn't - for now we'll avoid generating the module metadata file by disabling the relevant tasks

[simonbasle]

With @violetagg we've also noticed that the pom description is hardcoded and so isn't terribly relevant for the jblockhound-junit-platform submodule. I've used this PR as an opportunity to fix that.

cc @bsideup