Replace planet requires with local requires of main.rkt.

Otherwise this package's implementation would be using the planet
package -- and causing users of this package to download the planet
package -- which doesn't seem right.

Furthermore, my understanding is that it's preferable within a package
to use a relative require -- like `(require "main.rkt")` -- instead of
requiring the collection itself -- like either of `(require describe)`
or `(require (planet williams/describe/describe))`. So even if this were
still a planet package, I think this would be a good change to make.

describe/describe-test.rkt

@@ -16,8 +16,8 @@
 ;;; along with this program.  If not, see <http://www.gnu.org/licenses/>.
 ;;;
 
-(require racket/mpair)
-(require (planet williams/describe/describe))
+(require racket/mpair
+         "main.rkt")
 
 ;;; Booleans
 (printf "~n--- Booleans ---~n")

describe/float-test-double.rkt

@@ -1,6 +1,6 @@
 #lang racket
 
-(require (planet williams/describe/describe))
+(require "main.rkt")
 
 (printf "--- Using literals (double precision) ---~n")
 (describe 0.1)

describe/float-test-extended.rkt

@@ -1,7 +1,7 @@
 #lang racket
 
-(require racket/extflonum)
-(require (planet williams/describe/describe))
+(require racket/extflonum
+         "main.rkt")
 
 (printf "--- Using literals (extended precision) ---~n")
 (describe 0.1t0)

describe/float-test-single.rkt

@@ -1,6 +1,6 @@
 #lang racket
 
-(require (planet williams/describe/describe))
+(require "main.rkt")
 
 (printf "--- Using literals (single precision) ---~n")
 (describe 0.1s0)