Combine STG free variable traversals (#17978)

Previously we would traverse the STG AST twice looking for free variables.
 * Once in `annTopBindingsDeps` which considers top level and imported ids free.
   Its output is used to put bindings in dependency order. The pass happens
   in STG pipeline.
 * Once in `annTopBindingsFreeVars` which only considers non-top level ids free.
   Its output is used by the code generator to compute offsets into closures.
   This happens in Cmm (CodeGen) pipeline.

Now these two traversal operations are merged into one - `FVs.depSortWithAnnotStgPgm`.
The pass happens right at the end of STG pipeline. Some type signatures had to be
updated due to slight shifts of StgPass boundaries (for example, top-level CodeGen
handler now directly works with CodeGen flavoured Stg AST instead of Vanilla).

Due to changed order of bindings, a few debugger type reconstruction bugs
have resurfaced again (see tests break018, break021) - work item #18004 tracks this
investigation.

authors: simonpj, nineonine

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