re-write logic

Signed-off-by: Kevin Cui <bh@bugs.cc>

lib/filesystem.js

@@ -99,13 +99,13 @@ class Filesystem {
   }
 
   insertLink (p) {
-    const realSrcPath = fs.realpathSync(this.src)
-    const target = path.join(path.dirname(p), fs.readlinkSync(p))
-    const relativePath = path.relative(realSrcPath, fs.realpathSync(target))
-    if (relativePath.startsWith('..')) {
-      throw new Error(`${p}: file "${relativePath}" links out of the package`)
+    const symlink = fs.readlinkSync(p)
+    // /var => /private/var
+    const parentPath = fs.realpathSync(path.dirname(p))
+    const link = path.relative(fs.realpathSync(this.src), path.join(parentPath, symlink))
+    if (link.startsWith('..')) {
+      throw new Error(`${p}: file "${link}" links out of the package`)
     }
-    const link = path.relative(this.src, target)
     const node = this.searchNodeFromPath(p)
     node.link = link
     return link