store tags as a dir of files, not a file of lines

Currently we store all tags in a file (named `tags`) in the Issue dir.
The `tags` file has a line per tag.

This is bad because `ent sync` will get a git merge conflict if we add
or remove a tag near where someone else added or removed a tag.

Better to have a `tags` a directory, with an empty file per tag.
The filename is the tag.  This use is conflict free in git.