Unify engineDirs and toolsDir in Cache of Flutter Tools. (#7644)
Unify engineDirs and toolsDir in Cache of Flutter Tools. (#7644)

The zip files downloaded from cloud storage are not clearly separated
into engine artifacts and tools anyways.

Also, because of this unclear separation, we were downloading
`linux-x64/artifacts.zip` twice before.
Hixie/flutter,tvolkert/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,cbracken/flutter,turnidge/flutter,Hixie/flutter,Hixie/flutter,tvolkert/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,Hixie/flutter,Hixie/flutter,flutter/flutter,turnidge/flutter,cbracken/flutter,flutter/flutter,flutter/flutter,flutter/flutter,turnidge/flutter,turnidge/flutter,tvolkert/flutter,cbracken/flutter,tvolkert/flutter,cbracken/flutter,cbracken/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,turnidge/flutter,tvolkert/flutter,cbracken/flutter,tvolkert/flutter,cbracken/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter