[common] Ensure runfiles meet their promise of being absolute paths (R…

…obotLocomotion#17643)

* [common] Ensure runfiles meet their promise of being absolute paths

Add more cwd debug-logging to show what's happening.

Update the hardware_sim demo to set a valid runfiles path.

common/find_runfiles.cc

@@ -71,6 +71,7 @@ RunfilesSingleton Create() {
     result.runfiles.reset(Runfiles::Create(argv0, &bazel_error));
   }
   drake::log()->debug("FindRunfile mechanism = {}", mechanism);
+  drake::log()->debug("cwd = {}", filesystem::current_path());
 
   // If there were runfiles, identify the RUNFILES_DIR.
   if (result.runfiles) {
@@ -81,8 +82,10 @@ RunfilesSingleton Create() {
         // `bazel-bin/target`.
         // TODO(eric.cousineau): Show this in Drake itself. This behavior was
         // encountered in Anzu issue 5653, in a Python binary.
-        result.runfiles_dir =
-            filesystem::path(key_value.second).lexically_normal().string();
+        filesystem::path path = key_value.second;
+        path = filesystem::absolute(path);
+        path = path.lexically_normal();
+        result.runfiles_dir = path.string();
         break;
       }
     }

examples/hardware_sim/BUILD.bazel

@@ -73,6 +73,8 @@ sh_binary(
         "//manipulation/models/ur3e:prod_models",
         "//manipulation/models/wsg_50_description:prod_models",
     ],
+    # Use the runfiles of the ":homecart", not of the ":hardware_sim".
+    env = {"RUNFILES_DIR": "../"},
 )
 
 add_lint_tests()