When using get() method directly on PathItem the alias is not loaded

Problem/Motivation

This was an oversight in #2846554: Make the PathItem field type actually computed and auto-load stored aliases.

when using get directly on a PathItem and not a PathFieldItemList instance the alias is not loaded.

So this will not work:

$node_storage->resetCache();
$loaded_node = $node_storage->load($node->id());
$this->assertEquals('/foo', $loaded_node->get('path')[0]->get('alias')->getValue());

This was discovered in #2871591: Allow ComplexData in TypedData to specify computed properties that should be exposed in normalization and other contexts
because it caused the path values not to be normalized in that issue.

This issue fixes the normalization.

Proposed resolution

Override \Drupal\path\Plugin\Field\FieldType\PathItem::get() and call \Drupal\path\Plugin\Field\FieldType\PathItem::ensureLoaded before calling the parent get() method.

Remaining tasks

Patch & tests & Review

User interface changes

None

API changes

None

Data model changes

None