FFI docs: fix some missing links
net/git-checkout: support "dumb" protocol for discovery

Supporting just reference discovery can be useful for certain
testing configurations.
raco pkg {install,update}: add `--clone <dir>` mode

Using `--clone <dir>` with a Git-based package source causes the
package installation to be linked to a clone of the repository
as a subdirectory of <dir>. The package can be developed locally
in the usual way with Git tools, but `raco pkg update` can itself
pull updates to the package/repository.

See the new chapter 6 in "Package Management in Racket" for
more information.
raco pkg {install,update}: track Git repo source from a catalog

When a package is installed via a catalog, and the source provided
by the catalog is a Git repostory, then allow `--clone` to use
that repository when just the package name is given.
net/git-checkout: finish support for "dumb" HTTP(S)

Full dumb-server support is even more useful for testing.
raco pkg {install,update}: help users keep repo clones consistent

When packages A and B have the same Git repository source, and
when at least one of them is linked as a repository clone, then
the package system doesn't require A and B to both be linked or
both be linked to the same clone... but that can lead to confusion
for users. The package manager now detects this situation and in
many situations can update non-clones to make then clone links
at the same repository.