Follow-up from "Fix TH dependencies (#22229)" to refactor TH module structure

In !12456 (comment 560335), @clyring proposed the following refactoring (nearly; I slightly changed the module names involved to use the TH.Internal.* namespace instead of TH.Lib.*)

flowchart TD

subgraph A[Language.Haskell.TH.Syntax]
P[A bunch of re-exports]
end

subgraph B[Language.Haskell.TH.Internal.Lift]
Q[The Lift typeclass]
R[Various Lift instances]

P --> R
P --> Q
R --> Q
end

subgraph C[Language.Haskell.TH.Internal.Lib]
S[A bunch of stuff used in TemplateHaskellQuotes desugaring]
U[The Quote typeclass]

U -. co-located with (referring to Note [Tracking dependencies on primitives]) .- S
Q --> U
P --> U
end

subgraph D[Language.Haskell.TH.Internal.Syntax]
T[The main template-haskell AST types]

P --> T
Q --> T
R --> T
S --> T
U --> T
end

A -- imports --> B
B -- imports --> C
C -- imports --> D

The reason for this refactoring is that currently, we declare any use that imports Language.Haskell.TH.Lib.* but not Language.Haskell.TH.Syntax as unsupported, which is an unnecessary pitfall that would be fixed by the proposed refactoring.