capture: rename PidRoot to Overlay and add src/dst

Really what we want to deal with here is tracking an overlay that we may
need to be able to decode after the fact (in case processes exit or we
need to do post-processing symbol resolution).

For the podman case, that is $some_path mapped to root (/), generally
speaking. For flatpak though, that would have two mappings, one for
/app and another for /usr (possibly more).

src/libsysprof/sysprof-proc-source.c

@@ -259,12 +259,9 @@ sysprof_proc_source_populate_pid_podman (SysprofProcSource *self,
                                                     info->layers[i],
                                                     "diff",
                                                     NULL);
-          sysprof_capture_writer_add_pid_root (self->writer,
-                                               SYSPROF_CAPTURE_CURRENT_TIME,
-                                               -1,
-                                               pid,
-                                               path,
-                                               i);
+          sysprof_capture_writer_add_overlay (self->writer,
+                                              SYSPROF_CAPTURE_CURRENT_TIME,
+                                              -1, pid, i, path, "/");
         }
     }
 }