v0.0.0-8541-g8ccafe338

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 https://github.com/google/xls/commit/f79c1dcd5f3a574d42081208ca8cdf9162615653.
2. rules_hdl ported it in https://github.com/hdl/bazel_rules_hdl/commit/1c23debf2cc69ef62717da1085dba5ac1e66271a
3. XLS ported a patch with an implicit dependency on rules_hdl in https://github.com/google/xls/commit/6bd8019a72b3e2dda74e435b12a65746f73d91ca.

PiperOrigin-RevId: 799357631