Revert my ignorance; rename 'test' to standard
MakeMaker 't' folder.
Revert further ignorance; flatten the 'lib' directory
to match correct module structures and move 'plugins'
back to the root.
Various corrections and cleanup

* rename SHARE_DEST to IKIWIKI_SHARE
* add 'basewiki' explicitly (since it's not part of my
  'underlays' -- questionable ...)
* fix the restructured 'plugins' build
* other minor formatting / style
Add 'json' version of MYMETA to the ignore file.
Correct 'locale' makefile after reverting to
standard 'MakeMaker' structure (i.e. correct
'lib' location, removing the 'perl5' component)
Correct errors in 'install' components, specifically

* basewiki
* templates
* themes

where these were either incomplete or incorrectly located.