merge MetaSearch into QGIS core

cc @kalxas @rduivenvoorde @alexbruy @timlinux @jef-n 

This PR officially moves MetaSearch into QGIS core as per https://github.com/geopython/MetaSearch/issues/62.  Notes:

- workflow for out of band MetaSearch releases are TBD, though it might be easiest if https://github.com/geopython/QGIS be the place to cut such releases from.  `pavement.py` is not part of the build but will be adjusted / used for out of band releases to https://plugins.qgis.org Needs further discussion
- there are existing translations in Transifex at:
 - https://www.transifex.com/projects/p/MetaSearch/resource/plugin_core/
 - https://www.transifex.com/projects/p/MetaSearch/resource/plugin_templates/
...which are not part of this PR.  Any ideas how to handle these?  n.b. docs have been ported to https://github.com/qgis/QGIS-Documentation, their Transifex translations need to ported as well