Strip out Rojo Target attributes on build if the target instance is in the build

[Corecii]

Rojo Target attributes are primarily used for cross-file ref property assignment. Within a single rbxm/rbxl file, they are generally unnecessary, but the (yet to be released) syncback feature assigns these for all ref properties.

From the user experience side, this can be confusing, especially when the attribute is out-of-date due to the actual ref property being updated by the user, but the attribute is out-of-date. The user also has no way to differentiate between important target attributes for cross-file refs and unimportant target attributes for in-file refs which will be overwritten.

By cutting out in-file target attributes, we improve the user experience: the only target attributes present in a build will be the attributes for items that are in a different file. The signifies to the user, "this is important; it is here for a reason."

---

Implementation: the easiest is to have middleware (or a similar step) delete all Target attributes which refer to items in the project. The only attributes left will be ones referring to instances outside of the project. This will work correctly for build and sync since the attribute is "not present" in the project after initial processing. This will not cause an issue for syncback because syncback will re-assign in-project ref target attributes using the actual ref property values.