Avoid unnecessary files.{rm_recursive,symlink} in symlinkWithOverwrite.

When we copy node_modules directories during rebuilds, we try to create
symlinks instead of doing a full recursive copy. Very often, however, the
symlinks already exist, and they point to the right location already, so
we can avoid recreating them for even better performance.

Another optimization identified by using the qualia:profile package
created by @veered: https://atmospherejs.com/qualia/profile

tools/fs/files.js

@@ -533,6 +533,11 @@ export function symlinkWithOverwrite(source, target) {
     files.symlink(source, target);
   } catch (e) {
     if (e.code === "EEXIST") {
+      if (files.readlink(target) === source) {
+        // If the target already points to the desired source, we don't
+        // need to do anything.
+        return;
+      }
       // overwrite existing link, file, or directory
       files.rm_recursive(target);
       files.symlink(source, target);