vname path should not start with "/", should incorporate root and corpus translation

[kamahen]

After discussion about why underhood didn't work, it turns out that the Kythe API expects path names to not start with "/".

The proper way of doing this is to have mappings from (corpus, root) to a spot in the file system and to pass (corpus,root,path) around wherever current "path" is used.

[kamahen]

Leaving this item open because the fix is a bit of a hack and a better fix should be done.