SQLite schema introspection should use PRAGMA instead of parsing to retrieve foreign keys

[Simon Charette]

The SQLite backend's DatabaseIntrospection.get_relations uses a complex combination of regexes to extract foreign key constraints from DDL while ​PRAGMA foreign_key_list works just fine as ​_get_foreign_key_constraints.

[Mariusz Felisiak]

​PR

[Mariusz Felisiak <felisiak.mariusz@…>]

In 30ec7fe:

Refs #33288 -- Made SQLite introspection raise DatabaseError on nonexistent tables.

All the other backends behave this way and we had to make adjustments
to our test suite to account for this discrepancy.

This also allows SQLite's get_relations() not to raise on a nonexistent
table while making sure the InspectDBTestCase.test_introspection_errors
test which ensures an error during introspection is surfaced in
generated models files still passes.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 483e30c3:

Fixed #33288 -- Made SQLite introspection use information schema for relations.

Previous solution was using brittle and complex parsing rules to
extract them from the SQL used to define the tables.

Removed a now unnecessary unit test that ensured the removed parsing
logic accounted for optional spacing.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 560ff988:

Refs #33288 -- Removed SQLite's DatabaseIntrospection._get_foreign_key_constraints().

The get_relations() method returns the exact same data in a more generic
format.