Cannot declare class because the name is already in use
At this point, after much playing around it seems to be working (at least on to the next bit of debugging).

To sum up, it was mostly changes to composer.json, followed by 

composer remove kr37/calendar37
composer require kr37/calendar37


src/Plugin.php became src/Calendar37.php
Added "class": "kr37\\calendar37\\Calendar37" to the "extra" part of composer.json
Corrected the autoload "psr-4" to "kr37\\calendar37\\": "src/"
Made sure that all the names matched up just right.
This error, "because the name is already in use," should really be "because the name doesn't match up."