Support staged Play applications

Play support the ability to package an application to be run "in
place". In that case the "start" script ends up in the "target"
directory and the Play JARs end up in "target/staged". So allow
"staged" as a synonym for "lib" in the Play framework detection logic.

[#51899933]
Merge 51899933-staged to master

[Completes #51899933]