Revert "allow composing (i.e. adding) FileCollections that don't implement MinimalFileSet" in favor of other implementation

This reverts commit 4270c283049f6394bc208e3acde63756f793d06b.
fixed sonar property names in sonar-runner samples
make DelegatingFileCollection implement MinimalFileSet
added specs to make sure that source class path feeding into inferred groovy/scala class path only gets resolved if/when the latter is actually used