Uniform handling of -w[ith-compiler]

[AndreasPK]

It would be nice if -w would be supported by all commands which require a GHC executable.

Currently at least sandbox and init don't support it.

[lspitzner]

• init "cabal init" should support support --with-ghc #4936
• exec cabal exec -w --with-compiler (generalization of #1899) #2177 (v2-exec supports -w already)
• freeze Add cabal freeze flag -w --with-compiler #2245 (v2-freeze supports -w already)
• fetch Add -w flag to cabal fetch #2242
• sandbox init (i think; haven't confirmed) (NB: sandboxes are deprecated in the v2-* model)
• gen-bounds

[hvr]

@lspitzner you forgot cabal gen-bounds (and in case cabal outdated learns to trigger the solver, maybe even that one) :-)