Mark a Python plugin without an __init__.py as broken

utils.findPlugins() checks for the existence of an __init__.py which
results in an attempt to enable a plugin without an __init__.py being
blocked with the only notification being a message in the Plugins log:

WARNING    Plugin "<name>" is not compatible with this version of QGIS.
             It will be disabled.

This is not very informative, especially because it is the same message
used for a metadata version mismatch.

Adding this check to Plugins.getInstalledPlugin(), which already
duplicates the metadata checks from utils.findPlugins(), results in the
plugin being marked as broken in the Plugins dialog.

(cherry picked from commit 051e4d1)

Author: cjmayo

python/pyplugin_installer/installer_data.py

@@ -600,6 +600,10 @@ def pluginMetadata(fct):
         errorDetails = ""
         version = None
 
+        if not os.path.exists(os.path.join(path, '__init__.py')):
+            error = "broken"
+            errorDetails = QCoreApplication.translate("QgsPluginInstaller", "Missing __init__.py")
+
         metadataFile = os.path.join(path, 'metadata.txt')
         if os.path.exists(metadataFile):
             version = normalizeVersion(pluginMetadata("version"))