Abolish the ghc-boot-th-next workaround

As of !12306 (closed), when bootstrapping the compiler we have a ghc-boot-th-next package. This package only exists during bootstrap, and its purpose is to allow usage of the in-tree ghc-boot-th library while the boot ghc-boot-th library (as a dependency of the boot template-haskell library) is also in scope.

Giving this package a distinct name seems to be currently necessary, but comes at the cost of a cognitive overhead. Ideally we would like to simply depend on ghc-boot-th-X or ghc-boot-th-(X+1).

When this package was wired-in/non-reinstallable it seemed unavoidable to have a workaround like this, but as of !12479 (merged) this is no longer the case.

On that branch I attempted to remove this workaround. See this thread: !12479 (comment 563083) But I ran into some errors.

Perhaps we need to wait for the boot compiler to have this patch, or maybe Hadrian needs to be taught about build plans with packages of the same name but different versions.