* Use more consistent language for Internal errors
There's no meaningful distinction between something being internal and
being 'marked as internal'.
* Rename property psalm_internal to internal in all storage classes
This property holds metadata that can be set using either @internal
or @psalm-internal in docblocks
* Change types of internal properties in storage from ?string to string
Simpler type is easier to handle. Non-internal methods can be considered
to be internal to the entire universe of PHP code, i.e. that code whose
namespace starts with the empty string. It's not a special case.
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