WIP - Dynamic dependencies for Haskell (discussion)

[avdv]

• Pass correct rpath origin setting to linker on Darwin
• Add cxx_merge_cpreprocessors_actions function with AnalysisActions parameter
• Add script to generate target metadata for Haskell
• Add ghc wrapper script which writes ABI hash and a deps file
• Add is_haskell_boot helper function
• Add HaskellToolchainLibrary provider
• Add haskell_toolchain_library rule
• Add generate_target_metadata and ghc_wrapper tool targets
• make compiler_flags be of arg type (#34)
• Add scripts to haskell rules
• Allow to declare dependencies for haskell sources manually
• Add get_source_prefixes helper function
• Make ghci_lib_path optional in template processor script
• Add attributes to haskell rules
• Support HaskellToolchainLibs for ide BXL
• Make cc toolchain tools optional
• Determine haskell dependencies and compile modules according to the dependency graph

IMPORTANT: Please don't raise pull requests here, but at facebook/buck2.

NOTE: this is a PR to get the discussion started on how to incorporate our changes into upstream.

I'll raise a PR at the buck2 repo soonish.

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These changes requires a few GHC patches:

• add -dep-json flag for emitting machine-readable dependency info https://gitlab.haskell.org/ghc/ghc/-/merge_requests/11994
• --show-iface-abi-hash https://gitlab.haskell.org/ghc/ghc/-/tree/wip/show-iface-abi-hash?ref_type=heads