[plt-scheme] no more "plthome.ss"

Continuing from the v301.13 announcement:

 http://list.cs.brown.edu/pipermail/plt-scheme/2006-April/012654.html

We have further purged the notion of "PLTHOME" in the current SVN
sources.

Instead of the "plthome.ss" library in "setup", a new "dirs.ss" library
provides functions like `find-main-collects-dir', `find-include-dir',
etc.

Installers still receive an argument in place of the old `plthome'
argument, but the argument is now the parent of the main "collects"
directory. If an installer needs to find, say, the "include" directory,
it should use `find-include-dir' instead of deriving a path from the
installer's argument.

For now, most directories are still found relative to the main
"collects" directory, but the paths will become configurable soon
(to allow configurable Unix installations).

Matthew