support file:// URLs as package sources
raco pkg install: add `--update-deps` flag

This option makes install and update even more consistent, also
`--auto` still implies `--update-deps` only in update mode.

Make `--update-deps' consult the user in `search-ask' mode,
make it ignored in `fail' or `force' mode.
racket/gui: adjust label size on some controls

Extra margins added by instantiating `message%' inside
`radio-box%', etc., made labels line up especially badly.
racket/gui: add 'disallow-invalid style to `get-text-from-user'
pkg: allow path string as `#:scope' argument
GUI package manager: fill in functionality

Add supoprt for various operations that became available in the
command-line interface.
protect package-source parsing against non-path strings