fix path in source-distribution README

Closes PR 14065
make installers: support Mac OS X ".pkg" format

The ".pkg"-based installer doesn't provide the option of picking
an installation path, but it can add a path in "/etc/paths.d"
so that users do not have to explicitly set the `PATH` environment
variable.
make site: add support for help pop-ups on generated download page
fix `runtime-require` duplicate path registration
fix "base" and "compatibility" test drivers