Do not remove sublibrary information when adding pvp-bounds

[dzhus]

This fixes stack sdist broken when using pvp-bounds, see
#5289. This is a
regression after #5156.

When using pvp-bounds we pretty-print an updated package description
back to .cabal file. If a set of sublibraries used in a dependency is
empty (from cabal-the-library perspective) then instance Pretty Dependency from
https://hackage.haskell.org/package/Cabal-3.0.0.0/docs/src/Distribution.Types.Dependency.html#line-58
will just print an empty set there:

build-depends: vector : {} <0.13

This leads to the following error when using stack sdist:

- 46:16:
unexpected Sublibrary dependency syntax used. To use this syntax the package needs to specify at least 'cabal-version: 3.0'. Alternatively, if you are depending on an internal library, you can write directly the library name as it were a package.
expecting space

What we want instead is to use the same sublibrary set as the original
dep. I suspect most of the time it's just [LMainLibName], which
thanks to instance Pretty Dependency omits the {} in the
dependency line and is hence (hopefully) compatible with whatever
cabal-version the original .cabal file used.

Note: Documentation fixes for https://docs.haskellstack.org/en/stable/ should target the "stable" branch, not master.

Please include the following checklist in your PR:

• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.

I tested this manually doing a stack sdist in a package which has pvp-bounds enabled.

[snoyberg]

Looks great, thanks! One request: can you rebase this to target the stable branch instead?

[snoyberg]

Thanks!