Disentangle BUCK caches for internally built and downloaded artifacts

Downloaded artifacts went directly into

  ~/.gerritcodereview/buck-cache

while artifacts built by BUCK went into the directory's `cache`
subdirectory:

  ~/.gerritcodereview/buck-cache/cache

. When just walking the file hierarchy on the command line, this
`cache` sub-directory is hardly visible, due to all the downloaded
artifacts. To increase visibility, we change to

* ~/.gerritcodereview/buck-cache/downloaded-artifacts
  (for downloaded artifacts)
* ~/.gerritcodereview/buck-cache/locally-built-artifacts
  (for artifacts built by buck locally)

That way `~/.gerritcodereview/buck-cache` (instead of one directory
and 100s files) now only contains two directories.

Change-Id: I7327ff6c1736f9af8ea15450949dc6a4bc10b684
diff --git a/.buckconfig b/.buckconfig
index 20954d7..7b75225 100644
--- a/.buckconfig
+++ b/.buckconfig
@@ -27,4 +27,4 @@
 
 [cache]
   mode = dir
-  dir = ~/.gerritcodereview/buck-cache/cache
+  dir = ~/.gerritcodereview/buck-cache/locally-built-artifacts