Move "no such module: VirtualSpatialIndex" code elsewhere

[simonw]

We currently show a useful warning (from #331) when the user tries to open a spatialite database without first loading the module:

datasette/datasette/app.py

Lines 547 to 554 in c692cd2

	except sqlite3.OperationalError as e:
	if e.args[0] == "no such module: VirtualSpatialIndex":
	raise click.UsageError(
	"It looks like you're trying to load a SpatiaLite"
	" database without first loading the SpatiaLite module."
	"\n\nRead more: https://datasette.readthedocs.io/en/latest/spatialite.html"
	)
	else:

This code is part of .inspect() which is going away - see #462 - so I need to find somewhere else for it to live.

[simonw]

This is a little bit tricky. This SQL looks like it may detect Spatialite tables:

select * from sqlite_master where type = "table"
and sql like "%CREATE VIRTUAL TABLE%"
and sql like "%USING VirtualSpatialIndex%"

But where to put it? I think this should go in a new "checks" mechanism, where we run checks against every connected database on Datasette startup.

[simonw]

To build a unit test for this I'm going to have to ship a small spatialite.db binary database as part of the git repo.

This is because I need the tests to run even when the spatialite module is not available - but you cannot create a spatialite database without having access to that module.

I'll include a build script in the repo for constructing that database.