stack_snapshot use executable components in tools attribute

[aherrmann]

Closes #1306

Uses executable components of package dependencies in the tools attribute in Cabal targets generated by stack_snapshot. E.g. we no longer need to build alex and happy outside of stack_snapshot to then pass them in via stack_snapshot's tools attribute. Instead, stack_snapshot will generate targets @stackage-exe//alex and @stackage-exe//happy and pass them into the tools attribute of any generated haskell_cabal_* target that depends on alex or happy respectively.

A use-case where this becomes very useful is the tool c2hs. Contrary to alex or happy it does itself depend on Hackage packages, e.g. language-c. Meaning, if someone wants to include a package that depends on c2hs in a stack_snapshot call, say grpc-haskell-core, then one has to first build c2hs separately. To achieve that one has to define a separate stack_snapshot for the dependencies of c2hs. This approach does not scale very well. With this PR the issue is avoided, as stack_snapshot will generate @stackage-exe//c2hs itself and pass it to grpc-haskell-core automatically.

The tools attribute to stack_snapshot is still available in case users do wish to provide external tools, e.g. a Nix provided alex binary. We keep an externally provided alex in rules_haskell's stack_snapshot as a test-case for this.

This also simplifies the start script, as it is no longer a common use-case to have to provide an externally built tool.

[aherrmann]

Tested on the daml repository, see digital-asset/daml@04f38dc.

[mboes]

This looks fine, but I don't see in the PR where these executables are being called. What does their label look like?

[aherrmann]

This looks fine, but I don't see in the PR where these executables are being called. What does their label look like?

The user facing labels are the ones generated by #1304, e.g. @stackage-exe//alex. These user facing labels are only generated for packages that are listed in the packages attribute. I.e. binaries of private packages are not exposed. Within stack_snapshot itself tools are referenced by this code using the internal labels of the form @stackage//:_alex_exe_alex. This is so that dependencies on executables of private packages work as well. These labels are not user facing.