A cleaned tree is now much closer to a pristine tree that has just
been checked out and never built. The only extra created files that
are left behind are ".gradle", "buildSrc/.gradle", and
"buildSrc/build".
This gets rid of some Eclipse warnings that stem from Buildship being
confused about what it should treat as a source directory if Maven and
Gradle are both being used in the same tree.
I was confused about the differences among:
srcDir 'foo'
srcDirs ['foo']
srcDirs = ['foo']
As it turns out, the first two append to the set of source
directories, while the last replaces this set entirely. I generally
want replacement, since WALA's current directory layout never matches
Gradle's assumed defaults.