This combination of changes moves the decision about rebuilding
"scribblings/main" and "scribblings/main/user" to `raco setup',
which is in a better position to know whether documentation should
be built at all.
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.
