pmaports.find: show extra-repos/systemd hint

If the user tries to build a package, but it can't be found: check if systemd is disabled and the package is in extra-repos/systemd, display a hint about it.

$ pmbootstrap build systemd
[17:37:02] NOTE: The package 'systemd' exists in extra-repos/systemd, but systemd is currently disabled
[17:37:02] ERROR: Could not find package 'systemd' in pmaports

Fix #2398 (closed)

Details are in the commit messages.

pmb/core/test_pkgrepo.py

@@ -19,6 +19,7 @@ def test_pkgrepo_pmaports(pmaports, monkeypatch):
 
     # Disable results caching
     pkgrepo_paths.cache_disable()
+    pkgrepo_default_path.cache_disable()
 
     paths = pkgrepo_paths()
     print(f"[master] pkgrepo_paths: {paths}")
@@ -37,7 +38,10 @@ def test_pkgrepo_pmaports(pmaports, monkeypatch):
         == 0
     )
 
-    paths = pkgrepo_paths()
+    paths = pkgrepo_paths(with_extra_repos="disabled")
+    assert len(paths) == 1
+
+    paths = pkgrepo_paths(with_extra_repos="enabled")
     assert len(paths) == 2
 
     # systemd is the first path, since we want packages there to take priority