Add support for unit-based hie-bios script for ghc 9.4+

Starting with GHC 9.4, GHC, and therefore HLS indirectly, will segregate
the command line flags for different compilation units if the flags for
that unit are placed in a file and passed as -unit @unitfile. GHC will
accept multiple unitfiles on the command line and keep the arguments
separate.

As an example of something this fixes, currently if you have to
haskell_library targets, A and B, and A depends on B and B imports
external library C, the current hie-bios script makes modules from C
visible for import into A, which will fail to compile unless A also
explicitly depends on C.

  A -> B -> C, but not A -> C (directly)

Add a multi flag to a repl. This currently doesn't affect the base repl
target, it only affects the hie-bios target that also gets generated.
This change would be a good basis for starting a repl with multiple unit
files as arguments.

For a multi-repl, we carefully segregate out what should be transitive,
and what should not be transitive. For example, package databases should
be transitive, but package ids should not be.

In order to correctly build the unit files for each unit, we collect the
haskell dependency graph as a part of _create_HaskellReplCollectInfo.
Then we can use this dependency graph to build up HaskellReplDepInfo and
HaskellReplLoadInfo for haskell targets from the HaskellReplLoadInfo
and HaskellReplDepInfo from the corresponding structs of their direct
dependencies. Because we are going to use some of these intermediate
products to produce the corresponding unit files, it is important that
we have more control as we build these up.

To build the unit files, we walk the haskell dependency graph in post
order. We build the load_info and dep_info as above, and if we determine
that this is a source-loaded dependency, then we pair the load_info and
dep_info into a HaskellReplInfo. Each HaskellReplInfo corresponds to a
unit file. The actual script just outputs "-hide-all-packages" and
"-unit @unitfile" for each of the unit files generated in this way.

Where possible, the filenames and include paths are output relative to
the workspace root. This is done because the editor, and hls, are
expected to be running in the workspace root, and not in the execution
root. Anything that is a generated file, or a directory (e.g. an import
directory) for generated files must be prefixed with the execution root.
The execution root is unavailable while creating an output. Bazel does
this so that particularities of a given machine don't leak out into the
outputs. In order to write unit files that contain the execution root,
the unit files can't be direct outputs of bazel rules. Instead, they are
outputs of a shell script, which is itself a rule output. For each of
the unit files, we produce a shell script fragment that is included by
the hie-bios shell script and writes to the corresponding unit file.

Author: iteratee

haskell/private/hie_bios_wrapper.sh

@@ -18,3 +18,10 @@ SCRIPT_ABS="$(rlocation %{OUTPUT_RLOCATION_PATH})"
 RULES_HASKELL_EXEC_ROOT=${SCRIPT_ABS%/"$SCRIPT_REL"}
 ARGS="%{ARGS}"
 echo "$ARGS"
+UNIT_FILE_FRAGMENTS="%{UNIT_FILE_FRAGMENTS}"
+for FRAGMENT in $UNIT_FILE_FRAGMENTS; do
+  UNIT_FILE=${FRAGMENT%.sh}
+  . "$FRAGMENT" >"$UNIT_FILE"
+  echo "-unit"
+  echo "@${UNIT_FILE}"
+done