R1:7075222b4b33

Validate paths before saving them when editing an owners package

Summary:
Paths in owners packages when referring to a directory should always end with
a trailing slash. (Otherwise, some things break, like loading the owning
packages for a path.) With this change, PhabricatorOwnersPackage now requires
that the path provided for a package is valid, and if the path is for a
directory, it adds a trailing slash if one was not provided.

Test Plan:
Edited a path in a package and left off the trailing slash.