--load-extension=spatialite shortcut option

[simonw]

In conjunction with #135 - this would do the same thing as --load-extension=path-to-spatialite (see #134)

[simonw]

I think this should initialize SpatiaLite against the current database if it has not been initialized already.

Relevant code: https://github.com/simonw/shapefile-to-sqlite/blob/e754d0747ca2facf9a7433e2d5d15a6a37a9cf6e/shapefile_to_sqlite/utils.py#L112-L126

def init_spatialite(db, lib):
    db.conn.enable_load_extension(True)
    db.conn.load_extension(lib)
    # Initialize SpatiaLite if not yet initialized
    if "spatial_ref_sys" in db.table_names():
        return
    db.conn.execute("select InitSpatialMetadata(1)")


def ensure_table_has_geometry(db, table, table_srid):
    if "geometry" not in db[table].columns_dict:
        db.conn.execute(
            "SELECT AddGeometryColumn(?, 'geometry', ?, 'GEOMETRY', 2);",
            [table, table_srid],
        )

Not sure if I should add a utility function or CLI command for that ensure_table_has_geometry bit.

[simonw]

The --spatialite option should be available for other useful commands too, refs #137.

[simonw]

To keep the code cleaner, I'm tempted to support this instead:

--load-extension=spatialite

Where spatialite is a special shortcut value that triggers a search for that module in known locations.

Users could still load a module in a file called spatialite in the current directory using:

--load-extension=./spatialite

In fact, --load-extension=spatialite could handle that case too by always checking for a file called spatialite before attempting to search for it in known locations.