Remove boost dependency from XLS.

It is just replicating the dependency from `@rules_hdl` and depends on the `@com_github_libbacktrace` dep being a transitive dependency.

Instead, we just use the one provided by rules_hdl. For this to work, `rules_hdl_dependency_support` needs to be invoked in WORKSPACE before loading of `rules_hdl_init`. This ordering was wrong before and also, cannot happen in a .bzl which loads both together. Since workspace macros are discouraged for hybrid bzlmod mode anyway (https://bazel.build/external/migration#hybrid-mode), we remove `initialize_external.bzl` entirely.

Historical footnote: we ended up with two copies after:
1. XLS added boost for nextpnr in f79c1dc.
2. rules_hdl ported it in hdl/bazel_rules_hdl@1c23deb
3. XLS ported a patch with an implicit dependency on rules_hdl in 6bd8019.

PiperOrigin-RevId: 799357631

Author: mikex-oss

WORKSPACE

@@ -31,6 +31,18 @@ load(
 # https://github.com/bazelbuild/rules_python/issues/1560#issuecomment-1815118394
 py_repositories()
 
-load("//dependency_support:initialize_external.bzl", "initialize_external_repositories")
+load("@rules_hdl//dependency_support:dependency_support.bzl", rules_hdl_dependency_support = "dependency_support")
 
-initialize_external_repositories()
+rules_hdl_dependency_support()
+
+load("@rules_hdl//:init.bzl", rules_hdl_init = "init")
+
+rules_hdl_init(python_interpreter_target = "@project_python_host//:python")
+
+load("@rules_7zip//:setup.bzl", "setup_7zip")  # needed by rules_hdl
+
+setup_7zip()
+
+load("//dependency_support/llvm:initialize.bzl", initialize_llvm = "initialize")
+
+initialize_llvm()

dependency_support/boost/BUILD.bazel

@@ -19,7 +19,6 @@
 # Eventual goal that none of this is needed anymore and the file can be removed.
 
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
-load("//dependency_support/boost:workspace.bzl", repo_boost = "repo")
 load("//dependency_support/llvm:workspace.bzl", repo_llvm = "repo")
 load("//dependency_support/pprof:workspace.bzl", repo_rules_pprof = "repo")
 load("//dependency_support/rules_hdl:workspace.bzl", repo_rules_hdl = "repo")
@@ -39,7 +38,6 @@ def load_external_repositories():
     # https://github.com/google/xls/issues/931#issue-1667228764 for more
     # information / background.
 
-    repo_boost()
     repo_llvm()
     repo_rules_hdl()
     repo_rules_pprof()