Only munge internal dependencies when printing when there is no explicit syntax

* In `postProcessInternalDeps` the shadowing logic which existed prior
  to cabal format 3.4 is implemented in a post processing step.

  The algorithm there replaces any references to internal sublibraries
  with an explicit qualifier. For example if you write..

  ```
  library
    build-depends: foo

  library foo
    ...
  ```

  this is reinterpreted as

  ```
  library
    build-depends: mylib:foo

  library foo
    ...
  ```

* In `preProcessInternalDeps` the inverse transformation takes place,
  the goal is to replace `mylib:foo` with just `foo`.

* Things go wrong if you are using version 3.0 for your cabal file
  because
  - In 3.0 the qualifier syntax is introduced so you can be expliciit
    about sublibrary dependencies
  - The shadowing semantics of non-qualified dependencies still exists.

  So the situation is that the user is explicit about the sublibrary

  ```
  library

  library qux
    build-depends: mylib:{mylib, foo}

  library foo
  ```

  1. Post-process leaves this alone, the user is already explicit about
     depending on a sublibrary.
  2. Pre-processing then rewrites `mylib:{mylib, foo}` into two
     dependencies, `mylib` and `foo` (no qualifier).
  3. When parsed these are two separate dependencies rather than treated
     as one dependency, roundtrip test fails.

Solution: Only perform the reverse transformation when the cabal library
version is <= 3.0 and doesn't support the explicit syntax.

Now what happens in these two situations:

1. ```
   library
     build-depends: foo

   library foo
     ...
   ```

  this is reinterpreted as

  ```
  library
    build-depends: mylib:foo

  library foo
    ...
  ```

  then printed and parsed exactly the same way.

2. Explicit syntax is parsed and printed without being munged (when
   supported)

Note: Mixins only supported sublibrary qualifiers from 3.4 whilst
dependencies supported this from 3.0, hence the lack of guard on the
mixins case.

Fixes #10283