Rationalise `Package` type's `packageDeps`, `packageAllDeps` and/or `packageSubLibDeps` fields

[mpilgrem]

This arises out of a code comment in #5659. The data constructor of the Package type now includes fields:

, packageDeps :: !(Map PackageName DepValue)
  -- ^ Packages that the package depends on, both as libraries and build tools.
, packageAllDeps :: !(Set PackageName)
  -- ^ Original dependencies (not sieved).
, packageSubLibDeps :: !(Map MungedPackageName DepValue)
  -- ^ Original sub-library dependencies (not sieved).

The question is: is some or all of that duplicative and capable of rationalisation? The answer to that question is likely downstream of solving the residual problem of getting sub-library dependencies to work (eg #5839).

[theobat]

We should do the same in LocalPackage but closing this one for now as most of it is done