Also, more consistently propagate a given checksum, which can
happen more through the `pkg-install' export from `pkg/lib'
than through `raco pkg'.
Also, report to the user when consulting GitHub or downloading
a checksum.
Adds `pkg/pnr-db', `raco pkg index-copy', and `raco pkg index-show'.
Includes tools to build a database of modules that are supplied by
packages, which will be useful for a tool to recommend package
installs when a module is not found.
Also, document `pkg/lib' and add extra helper functions
for getting package information from a package name resolver.
