Revert "Always use trailing forward slash on plugin dir"

This reverts commit 0c2c34addce91698b475d404ceff9fd99cd86f8a.
Add maktaba#path#AsDir to canonicalize dir paths
Change maktaba#rtp#Join to strip trailing slashes

Also change maktaba#rtp#Split to canonicalize all paths returned, and
in the process fix minor bugs in Add and Remove where path comparisons
would false-negative because paths weren't canonicalized.
Merge pull request #79 from google/rtp_tailslash

Change maktaba#rtp#Join to strip trailing slashes