raco pkg update: extra tests
fix a test property
raco pkg update: `implies` package dependencies are always updated

.. where "always" means "unless `--ignore-implies` is specified".
raco pkg: adjust error messages that include command-line flags

A new `#:from-command-line?` argument to various functions indicates
whether error messages should try to suggest command line flags (since
the suggested flags do not make sense for other contexts, such as the
GUI package manager).
make snapshot-site: don't treat removed builds as failing