AbstractSource::$data is an untyped array, causing mixed-* fallout in downstream analyzers

[tyrsson]

Context

Found while working phpdb-mysql#60 (mago analyze fixes).

Data

Metadata\Source\AbstractSource::$data is declared protected array $data = []; with no shape (src/Metadata/Source/AbstractSource.php:58). Every keyed read/write against it in downstream sources (e.g. phpdb-mysql's Metadata\Source) resolves to mixed, producing mixed-array-assignment, possibly-invalid-argument, reference-constraint-violation, and too-many-arguments findings in static analysis wherever $data is touched.

@simon-mundy has the most context on this part of the metadata/SQL handling.