fix: avoid recreating existing, valid symlinks

This is a somewhat lazy approach to fixing an issue where pnpm
regenerates symlinks, even if the existing symlink is perfectly fine.
This behavior (of clobbering valid symlinks) can have adverse downstream
effects, like triggering watchers and invalidating caches (the mtime
changes).

This is not the ideal solution, because it incurs the cost of a failed
`readlink` if the symlink does _not_ currently exist (which is always
the case in a fresh pnpm install, and _sometimes_ the case in a pnpm
install that needs to move things around).

But in the happy case—when the node_modules are already mostly correct,
at Discord, we saw a 0.2-0.5s speedup in `pnpm install`. So I think it's
worth doing this, and then considering this function more holistically
if we find time.

src/index.ts

@@ -42,6 +42,15 @@ async function forceSymlink (
     renameTried?: boolean
   }
 ): Promise<{ reused: boolean, warn?: string }> {
+  // The happy path: the symlink already exists, and is correct (which is true
+  // in most already-initialized projects).
+  try {
+    const actualPath = await fs.readlink(path)
+    if (actualPath === target) {
+      return { reused: true }
+    }
+  } catch (e) { /* logic below handles all other cases */ }
+
   try {
     await fs.symlink(target, path, symlinkType)
     return { reused: false }