Relax restrictions on MAP_* constant names

[GriffinRichards]

Currently Porymap requires that for any map FooBar its ID is MAP_FOO_BAR. There's not much of a reason to require this, beyond the fact that we'd have to discover map IDs beforehand by opening every map.json file. We're already doing this in order to get MAPSEC and LAYOUT ids for the map list (and not caching the result!), so we may as well get all 3 of these constants on launch and relax the name restriction so that the only requirements are 1. it's unique, and 2. it begins with the user-provided prefix (MAP_ by default).