inside: document scheme_fd_to_semaphore()
add `exn:fail:{filesystem,syntax}:missing-module'

These exception types are intended as a hook for a programming
environment to recommend packages that can provide a module that isn't
found (through an uncaught-exception handler). The new exceptions are
generated by the default module name resolver and defaultload
handler. In addition to the exceptions, there's a new
`prop:exn:missing-module' property and `exn:missing-module?', which is
what an environment should use to detect relevant exceptions.

There's also a new `current-module-path-for-load' parameter, which
just provides a commuincation path from the module name resolver to
the the load handler. The parameter is a relatively ugly piece of the
puzzle, but it's the best I could find to squash the new functionality
into the existing stack of handlers.
pkg/lib: add `#:quiet?' options and adjust printing
pkg/pnr-db: add `get-pkgs-without-modules'

Also, by default, clear out information for old checksums when
setting a package's information.
pkg/lib: fix `get-pkg-content'

When discovering modules, handle the case that a directory
name doesn't work as a collection name.
pkg/pnr-db: fir and document `get-module-pkgs'
pkg/lib: add `pkg-index-update-local' and `pkg-index-suggestions-for-module'

Assembles other functions to create and consult a local database of
packages and the modules that the packages implement.