Problem/Motivation
After path aliases are converted to a content entity type, we should be able to remove the custom PathItem field type and replace it with a regular entity reference field targeting the new path_alias entity type.
Proposed resolution
Change the relationship between nodes/terms/media items and path aliases to an entity reference one.
Removing the custom path alias related code from various entity forms will most likely need a simple version of Inline Entity Form in core.
Remaining tasks
TBD.
User interface changes
Nope.
API changes
Yes, most likely for REST APIs: getting the path alias for a node will be done by requesting the referenced path alias entity.
Data model changes
A new base field will be added to entity types that support path aliases, currently nodes, taxonomy terms and media items.