Avoid depending on output of rustc

There is cyclic dependency between rustc and cargo which makes it
impossible to adjust cargo's expectations on rustc without leaving
broken commits in rustc and cargo.

tests/testsuite/artifact_dep.rs

@@ -391,7 +391,7 @@ fn features_are_not_unified_among_lib_and_bin_dep_of_different_target() {
             "d1/src/lib.rs",
             r#"
             #[cfg(feature = "d2")]
-            extern crate d2;
+            pub extern crate d2;
         "#,
         )
         .file(
@@ -564,18 +564,15 @@ fn build_script_with_bin_artifact_and_lib_false() {
             "bar/src/lib.rs",
             r#"
             pub fn doit() {
-               panic!("cannot be called from build script due to lib = false");
+               panic!("sentinel");
             }
         "#,
         )
         .build();
     p.cargo("build -Z bindeps")
         .masquerade_as_nightly_cargo()
         .with_status(101)
-        .with_stderr_contains(
-            "error[E0433]: failed to resolve: use of undeclared crate or module `bar`",
-        )
-        .with_stderr_contains(" --> build.rs:3:16")
+        .with_stderr_does_not_contain("[..]sentinel[..]")
         .run();
 }
 
@@ -598,7 +595,7 @@ fn lib_with_bin_artifact_and_lib_false() {
         .file(
             "src/lib.rs",
             r#"
-            fn foo() {
+            pub fn foo() {
                bar::doit()
             }"#,
         )
@@ -608,18 +605,15 @@ fn lib_with_bin_artifact_and_lib_false() {
             "bar/src/lib.rs",
             r#"
             pub fn doit() {
-               panic!("cannot be called from library due to lib = false");
+               panic!("sentinel");
             }
         "#,
         )
         .build();
     p.cargo("build -Z bindeps")
         .masquerade_as_nightly_cargo()
         .with_status(101)
-        .with_stderr_contains(
-            "error[E0433]: failed to resolve: use of undeclared crate or module `bar`",
-        )
-        .with_stderr_contains(" --> src/lib.rs:3:16")
+        .with_stderr_does_not_contain("[..]sentinel[..]")
         .run();
 }