Get only "SRID" CRS identifier (not authority+identifier) for GeoPackage layers

[kannes]

While for PostgreSQL/PostGIS and MSSQL ST_SRID resp. STSrid are used to get a simple integer EPSG code:

qgis-discovery-plugin/Discovery/mssql_utils.py

Line 113 in f652251

[%s].STSrid AS epsg,

qgis-discovery-plugin/Discovery/dbutils.py

Line 179 in f652251

ST_SRID("%s") AS epsg,

For GeoPackage the layer's CRS's authid() is used, which is a full string with authority identifier and code (e.g. "EPSG:4326"):

qgis-discovery-plugin/Discovery/gpkg_utils.py

Line 53 in f652251

epsg = layer.crs().authid()

This breaks the transformation in select_result() as QgsCoordinateReferenceSystem.fromEpsgId(foo) is used, which takes only the integer identifier.

qgis-discovery-plugin/Discovery/discoveryplugin.py

Line 383 in f652251

transform = QgsCoordinateTransform(QgsCoordinateReferenceSystem.fromEpsgId(src_epsg),

This PR simply strips "EPSG:" from the GeoPackage's auth ID. This won't work with non-EPSG CRSses but then I am not sure what would happen in PostGIS/MSSQL in those cases. It's an improvement for sure as it makes things work again.

Fixes follow-up comments in #95

[kannes]

Sorry for the force-push mess. I also added a quick check and warning message if a GPKG is not using an EPSG CRS. I was not brave enough to attempt to make the code around PG & MSSQL provide full CRS authority+identifier strings. ;)