[development] Move all core modules into their own directories.

[Nedjo Rogers]

> Maybe this makes for a good starting point to experiment with some of
> the ideas.  Execute from the command line in Drupal's root directory.

Useful. In addition, we should consider moving all a module's associated 
files into its directory. A core example is upload.js from /misc to 
/modules/upload (or a subdirectory).

This change would help us get away from our sometime practice of giving core 
components special treatment that e.g. contrib modules don't have access to.

(An example of this problem is the hard-coding of js behaviours into form 
element theme functions, e.g., autocomlete in theme_textfield(), see this 
issue http://drupal.org/node/57865. Obviously, contrib modules wishing to 
add behaviours to existing elements can't follow what's done in core.)