move `planet/resolver' back to core

In consultation with Sam, partially reverts efba034e01. The default
module name resolver uses `planet/resolver' for the build-in
`(planet ....)' form. The `raco planet' command is still in
"planet-lib", though.

pkgs/planet-lib/planet/lang/reader.rkt

@@ -1,6 +1,6 @@
 #lang racket/base
 (require syntax/module-reader
-         "../resolver.rkt")
+         planet/resolver)
 
 (provide (rename-out [planet-read read]
                      [planet-read-syntax read-syntax]

pkgs/planet-lib/planet/util.rkt

@@ -4,9 +4,9 @@
          planet/planet-archives
          
          planet/private/planet-shared
-         "private/linkage.rkt"
+         planet/private/linkage
          
-         "private/resolver.rkt"
+         planet/private/resolver
          "private/version.rkt"
          
          net/url