Migrate to Bzlmod

[jeltevanbommel]

I couldn't build the bootstrapper with Bazel anymore, getting errors saying that our current WORKSPACE-based system for external dependencies is deprecated.

ERROR: Skipping '//:bootstrapper': error loading package '': Unable to find package for @@[unknown repo 'io_bazel_rules_go' requested from @@]//go:def.bzl: The repository '@@[unknown repo 'io_bazel_rules_go' requested from @@]' could not be resolved: No repository visible as '@io_bazel_rules_go' from main repository. Was the repository introduced in WORKSPACE? The WORKSPACE file is disabled by default in Bazel 8 (late 2024) and will be removed in Bazel 9 (late 2025), please migrate to Bzlmod. See https://bazel.build/external/migration.

With --enable-workspace=true I got

bazel build //:bootstrapper --enable_workspace=true

WARNING: WORKSPACE support will be removed in Bazel 9 (late 2025), please migrate to Bzlmod, see https://bazel.build/external/migration.

ERROR: error loading package '@@com_google_protobuf//': Label '@@rules_pkg//pkg:mappings.bzl' is invalid because 'pkg' is not a package; perhaps you meant to put the colon here: '@@rules_pkg//:pkg/mappings.bzl'?

This PR migrates from WORKSPACE to Bzlmod, allowing to build again.
This version successfully creates the debian package and bootstrapper binary when running make all.