Mark a Python plugin without an __init__.py as broken

[cjmayo]

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.

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[luipir]

LGTM, @borysiasty @alexbruy or @nyalldawson I can't find any test about plugin installer. can you confirm? If not, we can merge (as usual WFS tests are failing travis)

[borysiasty]

@luipir right, no tests are written yet :/
Thanks @cjmayo for fixing this quite annoying bug!