Fix symlink package spec validation (platformio#4870)

When validating symlink:// packages, use the specified symlink path,
the same as for file:// packages.  This fixes missing symlink packages
in 'pkg list' and reinstalling on every build.

Author: willmmiles

platformio/package/manager/base.py

@@ -281,9 +281,16 @@ def test_pkg_spec(pkg, spec):
         # external "URL" mismatch
         if spec.external:
             # local folder mismatch
-            if os.path.abspath(spec.uri) == os.path.abspath(pkg.path) or (
-                spec.uri.startswith("file://")
-                and os.path.abspath(pkg.path) == os.path.abspath(spec.uri[7:])
+            if (
+                os.path.abspath(spec.uri) == os.path.abspath(pkg.path)
+                or (
+                    spec.uri.startswith("file://")
+                    and os.path.abspath(pkg.path) == os.path.abspath(spec.uri[7:])
+                )
+                or (
+                    spec.uri.startswith("symlink://")
+                    and os.path.abspath(pkg.path) == os.path.abspath(spec.uri[10:])
+                )
             ):
                 return True
             if spec.uri != pkg.metadata.spec.uri: