raco setup: --tidy mode (as needed by `raco pkg remove') and more

When a collection disappears, then documentation and metadata
cross-references need fixing up based on everything that remains
available, even though other setup actions are confined to
collections that are specifically relevant to the packages. The
new `--tidy' mode takes care of that.

Package installation now also provides `--avoid-main' when working
with packages in a scope other than installation scope, and `raco
setup' now better respects `--avoid-main' to avoid creating
executables or re-running documentation.

Also, revise the `raco' documentation to better orient it toward the
package manager.

original commit: 51d48f3ab4

collects/setup/option-sig.rkt

@@ -11,6 +11,7 @@
      compiler-verbose
      clean
      compile-mode
+     make-only
      make-zo
      make-info-domain
      make-launchers
@@ -18,6 +19,7 @@
      make-user
      make-planet
      avoid-main-installation
+     make-tidy
      call-install
      call-post-install
      pause-on-errors