Use rackjure/base and racket/base.

This is what I attempted to do in commit d48e09e using only racket/base.
The problem, of course, was that some code uses rackjure's custom
`#%app` for applicable dictionaries. Changing to racket/base compiled
fine, but resulted in runtime errors.

What I needed was a `#lang rackjure/base`, which didn't exist yet. I
created it. Now I'm using it here for files that used `#lang rackjure`.

Files that were using `#lang racket` _can_ switch to `#lang
racket/base`. If they don't need rackjure's #%app, they can avoid its
overhead.

TL;DR:
`#lang racket`   => `#lang racket/base`   + explicit requires
`#lang rackjure` => `#lang rackjure/base` + explicit reuqires
Need rackjure v0.7+, which introduces rackjure/base.
Avoid 404 links to r-l.org docs for local pkgs.

In other words, `doc-uri` should return #f for locally-installed
packages that aren't part of the Racket main distribution and therefore
don't have docs on www.racket-lang.org.

Also, add typed/racket and typed/racket/base to the list of modules for
which we'll use their documentation as a "tie-breaker", just like racket
and racket/base.
Refine commit 3a5148a

Prefer, in order: racket/base, racket, typed/racket/base, typed/racket.

Also fix typo in comment.
Work around path->pkg existing only in Racket 6.0+
Have to skip some unit tests when Racket < 6.0.

See commit df26d34
Merge branch 'rackjure-base'

* rackjure-base:
  Have to skip some unit tests when Racket < 6.0.
  Work around path->pkg existing only in Racket 6.0+
  Refine commit 3a5148a
  Avoid 404 links to r-l.org docs for local pkgs.
  Need rackjure v0.7+, which introduces rackjure/base.
  Use rackjure/base and racket/base.