diskcache: Construct correct paths
Created by: efritz
A bug was introduced in incremental processing of symbols data https://github.com/sourcegraph/sourcegraph/pull/27932/files?w=1#diff-8b3a8c5193ef74d96d16a575971641affe279db976cc0926d4dda179fc4d305dR251 which added a directory component to the cache, but cleanup wasn't updated.