Add tests for maktaba#plugin#Enter and fix bugs (ftplugin, error msg)

Fixes obvious bugs in `maktaba#plugin#Enter()` with 'ftplugin' files and the messed up ERROR(CannotEnter) message Chip noticed in #19.

This still doesn't fix #19 (the brokenness for autoload/ subdirectories). It's going to be pretty tricky to set up proper path detection to figure out whether a file is in *any* autoload/ dir for *any* valid plugin. I'm not really sure it's worth supporting `maktaba#plugin#Enter()` in autoload files for all the effort it'll take. I'd be in favor of just removing it again..

Also, it was very broken for ftplugin files, and still isn't quite right in a corner case with unnamed buffers (#40).