Add support for postgres tables

[HughKelley]

Hi,

I saw here that non-file based data sources are ignored.

Support for postgres tables would be very helpful to me. Before starting to put together a PR I wanted to ask your opinion of that option generally (Would such a PR be well received) and see if there are any major obstacles to putting that together that you are already aware of.

thanks!

[tarot231]

To support virtual layers, the layer dictionary will be rewritten from a single file path to multiple data sources, including databases.

To store the database in GeoPackage, I was going to create PROVIDER_SCHEMA.gpkg and write the process of importing the layerName as table in database.py.

However, it may not be completed forever, as my needs are being met at this time (no database is needed).

One obstacle to supporting databases is differences among data providers. For example, ogr data provider do not provide schema names for the same PostGIS data.

If you are going to do a pull request, please make it for all databases, not just postgres.

[kewgis]

Support for PostGIS data would be very useful.