Add unstable option to only emit shared/crate-specific files

The intended use case is for docs.rs, which can now copy exactly the
files it cares about, rather than having to guess based on whether they
have a resource suffix or not. In particular, some files have a resource
suffix but cannot be shared between crates: https://github.com/rust-lang/docs.rs/pull/1312#issuecomment-798783688

The end goal is to fix https://github.com/rust-lang/docs.rs/issues/1327
by reverting https://github.com/rust-lang/docs.rs/pull/1324.

This obsoletes `--print=unversioned-files`, which I plan to remove as
soon as docs.rs stops using it.

src/librustdoc/lib.rs

@@ -527,6 +527,14 @@ fn opts() -> Vec<RustcOptGroup> {
         unstable("print", |o| {
             o.optmulti("", "print", "Rustdoc information to print on stdout", "[unversioned-files]")
         }),
+        unstable("emit", |o| {
+            o.optmulti(
+                "",
+                "emit",
+                "Comma separated list of types of output for rustdoc to emit",
+                "[unversioned-shared-resources,toolchain-shared-resources,crate-specific]",
+            )
+        }),
     ]
 }