gradle: extract simple task names from subproject tasks (#5704)

Currently, only tasks with complete subproject specifier are added to
.gradletasknamecache. Gradle commands can be called for all (sub-)projects
they are defined for, using their name as defined in the subproject, here
called "simple" task names. One example is "gradle clean".

This patch adds support for parsing out those "simple" task names from the list
of fully specified task names. The .gradletasknamecache file will contain
both the fully specified names, and the "simple" names for your autocompletion
pleasure.