Enable dependency fixer for missing InstancePlaceholder paths

[Flavelius]

Fixes #37818. Fixes #44720.

This treats placeholder paths as a dependency to invoke the regular ext_resource dependency fixer.
This also fixes file-reimport not retaining existing uids for resources that were renamed, and resource dependency renaming also not transitioning existing uids.

I chose to string-replace occurrences, as it was much more straightforward than parsing and rewriting everything and to not duplicate a lot of code in the process.

[Rindbee]

Em, is there any reason why the scene referenced by InstancePlaceholder cannot be a recorded external resource?

Make it like an external resource, with additional flags to mark it, and it will be easier to update. May break the format of tscn/res files.

[Flavelius]

Em, is there any reason why the scene referenced by InstancePlaceholder cannot be a recorded external resource?

Make it like an external resource, with additional flags to mark it, and it will be easier to update. May break the format of tscn/res files.

In my understanding the main reason is that this would create a hidden reference loop of the scene instance to itself which Godot currently cannot handle. I guess you could technically try to convert the path to a reference on save and transform it back on load but that would equally be a hack. But maybe I didn't completely understand your proposed solution.

Edit: and my initial reasoning was to not change the behavior and api of something whose reasoning I do not fully understand, but maybe you're right and it can be designed like a regular instance reference