pkg metadata: add `update-implies`

The `update-implies` declaration is half of `implies` (even though the
name is bigger): update of the package implies an update of another,
but a dependency on the package is no license to access the content
of the other.

This declaration is intended as an alternative to keeping version and
dependency declarations in sync for a pair of packages that are always
updated together, normally because the packages are in the same
repository. For example, a "-test" or "-doc" package might have an
`update-implies` declartion on the corresponding "-lib" package.
add `update-implies` to package "info.rkt"s
fix doc typo
raco pkg: add `--binary-lib` package stripping to remove docs

The `--binary-lib` mode is like `--binary`, but it also omits
documentation to form a "binary library package".

The `--binary-lib` flag and "X-lib" vs. "X-doc" approaches solve the
same problem with different trade-offs:

 * When a package is split into "-lib" and "-doc" packages, then it's
   easier install non-documentation parts, and it's possible to
   install them from source. A programmer has to work more to split
   the packages, however, and the library and its implementation must
   have separate sources (i.e., no or restricted in-source
   documentation).

 * When a package is just "X", then users can install a no-source,
   no-documentation version by specifying `--binary-lib`, but only
   when the package is available from some catalog and provider in
   built form (such as from a distribution site or a package-build
   service).

In the long run, I think that relying on `--binary-lib will be best
and typical for most packages. The "X-lib" plus "X-doc" approach
that's common in the current distribution's packages, meanwhile, will
likely stick around for basic packages that are commonly useful in
constrained settings (including the setting of a package-build
service).
annotate and check packages for build and binary modes

If "p" is available as a source package, which is typical, then `raco
pkg install --binary p` would strip away the build dependencies of "p",
so that "p" would not install properly.

This commit changes `raco pkg install` to look for an annotation on
the package and complain if the annotation is inconsistent with the
requested conversion: a binary package cannot be used as a source
package or vice versa. (A built package, as provided by a snapshot
site, can be used as any kind of package.)
package-system docs: add FAQ entry on installing without documentation

The entry is mostly about the new `--binary-lib` option, but it
also describes the "-lib", "-doc", etc., convention.