DrRacket: treat installed packages like "collects" directories

... for the purpose of "populate 'compiled' directories".
DrRacket: disable "compiled/drracket" during skipped-file load

If DrRacket decides to skip a file for "populate compiled",
then there may exist a file in "compiled/drracket", anyway,
or there may be such a file for some depenency of the skipped
file. Before this patch, that situation was considered to be a
broken installation, and things would go bad in the likely
case that the "compiled/drracket" files were out of date.

To avoid that problem, parameterize `used-compiled-file-paths'
to drop the DrRacket "populate compiled" target while loading
the skipped file.

This change sets up a more selective "populate compiled" where
a package's modules might switch between eligible and
ineligible for compilation by DrRacket.
remove accidentally added files
DrRacket: if any file in package X is open, treat X as "in development"

... for the purpose of "populate 'compiled' directories" --- but only if
the user has write permission for the package directory.

This change may or may not be a good idea. The idea is that installed
packages generally should be treated in the same way as the main
"collects" tree (e.g., avoiding debugging instrumentation), but if you
happen to be developing a package, then you want it treated like things
that are not in the main "collects" tree. So, how do you pick? Maybe
opening a file in the package is a good way to pick.