Publish source features on update site

I noticed that the current build only creates source bundles but no source features, despite the fact that the `tycho-source-feature-plugin` is mentioned in the POM. Alas, that plugin is missing an execution.

I fixed this together with an update to Tycho 0.18. After a `mvn clean verify` you can find an update site with source bundles in `com.ibm.wala-repository/target/repository`.

I am afraid, however, that the relevant part in the POM (`tycho-source-feature-plugin` and `tycho-p2-plugin`) is a bit verbose. Alas, this is the shortest way I know to make sure that the `p2-metadata` goal is executed after the `source-feature` goal when both are bound to the same phase. (This kind of subtle order dependency was exactlu what phases were designed to avoid originally, but apparently a typically Tycho build contains more steps than Maven has phases...) That being said, a dedicated "features" parent POM might help. See <https://git.eclipse.org/c/recommenders/org.eclipse.recommenders.git/tree/features/pom.xml> for an example.