make installers: save client logs in "build/log" instead of "build/drive"

Also, copy log files to the destination of `make site', and report
the current time more frequently.
split "unstable" package into the usual "-lib", "-doc", and "-test"

Otherwise, packages that depend on "unstable" end up depending
on "racket-test".
make site: add support for rendering PDF documentation
raco setup: send Scribble debug into to log as well as verbose
path->collects-path: repair for single-collection packages

Added an optional `#:cache' argument that is propagated to
`path->pkg', if necessary.
distro-build: support for adding READMEs

This is a first cut; the default "README" configuration will be
improved.
change `release-notes' "info.rkt" field to `release-note-files'

The change avoids a collision with PLaneT fields.

Also, add checking for the form of a `release-note-files' value.
find-relevant-directories: fix 'no-user mode
clean up places benchmark compilation