Remove deprecated prebuilt_dependencies

[Profpatsch]

In theory, prebuilt_dependencies should be deprecated and thus unnecessary, but going through the usage, non-trivial parts of rules_haskell still depend on it:

• HaskellHaddock passes it as argument to haddock_wrapper.sh. Does that mean haskell_import is not yet supported by the haddock generator, @mrkkrp?

rules_haskell/haskell/haddock.bzl

Lines 117 to 124 in 2b163f7

	mnemonic = "HaskellHaddock",
	progress_message = "HaskellHaddock {}".format(ctx.label),
	executable = ctx.file._haddock_wrapper,
	arguments = [
	prebuilt_deps,
	args,
	ghc_args,
	],

• HaskellProtoc uses it internally

rules_haskell/haskell/protobuf.bzl

Line 148 in 2b163f7

"prebuilt_dependencies": ctx.toolchains["@io_tweag_rules_haskell//protobuf:toolchain"].prebuilt_deps,

Concerning haskell_import, I’m confused as to which version should actually be used. We have the new one in import.bzl, but it’s not exported as far as I can see. But since haskell_import should be the replacement for prebuilt_dependencies, we should define which one to use (or throw one out). cc @regnat

[Profpatsch]

Okay, when going through the code with Théophane, we found that prebuilt_dependencies actually has two uses:

1. attribute in all haskell rules
2. internal mechanism to pass prebuilt stuff, also for haskell_import

Since we already deprecated the user-facing part, we can remove the attribute, but keep the internal usage for now.

The next step would be to finish new-style haskell_import and get rid of all that internal wiring and the HaskellPrebuiltPackageInfo provider as well, but that’s out of scope for this issue.