pmb.helpers.pmaports: search for provides in both root and subpackages

At the moment, "provides" are only checked in the root package and not
in subpackages of APKBUILDs. Fix this by looking through the subpackages
as well.

pmb/helpers/pmaports.py

@@ -116,9 +116,12 @@ def _find_package_in_apkbuild(args, package, path):
     if package in apkbuild["subpackages"]:
         return True
 
-    # Search for provides in package
-    apkpkgs = [apkbuild]
+    # Search for provides in both package and subpackages
+    apkpkgs = [apkbuild, *apkbuild["subpackages"].values()]
     for apkpkg in apkpkgs:
+        if not apkpkg:
+            continue
+
         # Provides (cut off before equals sign for entries like
         # "mkbootimg=0.0.1")
         for provides_i in apkpkg["provides"]: