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/build/_package.py

@@ -355,9 +355,7 @@ def process_package(
                     " try again"
                 )
             return []
-        raise RuntimeError(
-            f"{pkgname}: Could not find aport, and" " could not find this package in any APKINDEX!"
-        )
+        raise RuntimeError(f"{pkgname}: Could not find it in pmaports or any APKINDEX!")
 
     if arch is None:
         arch = pmb.build.autodetect.arch(base_apkbuild)