Allow the location of the installation-specific "links.rktd" file to
be specified in "config.rktd", and also allow extra link files to
be provided.
Allow the same for package directories.
The main file/directory in each case corresponds to the file/directory
that can be modified by an installation-scope install. Extra files
or directories in a search path supports constant links and libraries
that are shared across installations --- like "/usr/lib" versus "/lib".
The 1-second granularity of filesystem timestamps is not
good enough to deetct changes to the file --- especially
when `raco pkg' installs links and runs `raco setup' (in
the same Rcket process), in which case a newly installed link
might not be detected.
The longer-term repair is probably to add file-changed
events to Racket (based on `inotify' and similar interfaces
provided by OSes).
A collection declares release notes with a `release-notes' field in
"info.rkt".
The "doc" directory is now populated exclusively by generated content,
instead of having a static "release-notes" directory (and a "keep-dirs.rktd"
file to record the static directories).
Keep track of the out<n>.sxref timestamps, so that `raco setup'
can detect when the database is out of sync (instead of assuming that
it's always in sync with out<n>.sxref files).
The "racket" directory contains a pared-back version of the
repository, roughly.
The "pkgs" directory everything else in the repository, but
organized into packages.
