Move schema definitions back into .install files.

It really doesn't make much sense to have a whole other file name just for schema files; rather, it's easier for new developers to put all of the "databasey kinds of stuff" in one place, and it also maintains the schema definition along with any updates modifying the schema. Barry Jaspan, one of the creators of the schema patch has said that he supports this change: http://jaspan.com/schema-api-drupal-6#comment-461

I tested an install and enabling all the modules, and it even seems to work! Woah! More testing would be appreciated though. :)