build rules_nixpkgs_core compatibly in isolation

ideally we would already operate within `bzlmod` here, but we do not
even have all dependencies present in [BCR][bcr] as of today. it is
another can of worms detailed in [#181][181].

here we just isolate `rules_nixpkgs_core` as a proper self-contained
repository as far as possible.

[bcr]: https://github.com/bazelbuild/bazel-central-registry/tree/92bb87fc41d549a47297af67c4135111308b7b03
[181]: #181

core/.bazelrc

@@ -0,0 +1,35 @@
+build --host_platform=@rules_nixpkgs_core//platforms:host
+
+# NOTE: all of the following is copy-pasted from `/.bazelrc` and necessary to
+# make this repository work compatibly in isolation.
+
+build --crosstool_top=@nixpkgs_config_cc//:toolchain
+# Using toolchain resolution can lead to spurious dependencies on
+# `@local_config_cc//:builtin_include_directory_paths`. This needs to be
+# resolved before `--incompatible_enable_cc_toolchain_resolution` can be
+# recommended for `nixpkgs_cc_configure_hermetic`.
+# build --incompatible_enable_cc_toolchain_resolution
+
+build --javabase=@nixpkgs_java_runtime//:runtime
+build --host_javabase=@nixpkgs_java_runtime//:runtime
+build --java_toolchain=@bazel_tools//tools/jdk:toolchain_hostjdk8
+build --host_java_toolchain=@bazel_tools//tools/jdk:toolchain_hostjdk8
+
+# CI Configuration
+# ----------------
+common:ci --color=no
+build:ci --verbose_failures
+test:ci --test_output=errors
+
+# Use a remote cache during CI
+build:ci --bes_results_url=https://app.buildbuddy.io/invocation/
+build:ci --bes_backend=grpcs://cloud.buildbuddy.io
+build:ci --remote_cache=grpcs://cloud.buildbuddy.io
+build:ci --remote_timeout=3600
+# Avoid failures of the form `deadline exceeded after 14999958197ns DEADLINE_EXCEEDED`.
+# See https://github.com/tweag/rules_haskell/issues/1498.
+build:ci --keep_backend_build_event_connections_alive=false
+
+# User Configuration
+# ------------------
+try-import %workspace%/.bazelrc.local

core/WORKSPACE

@@ -1,2 +1,63 @@
-# only temporary for compatibility with `WORKSPACE` setup
-# TODO: remove when migration to `bzlmod` is complete
+# NOTE: only temporary for compatibility with `WORKSPACE` setup!
+# TODO: remove when migration to `bzlmod` is complete
+
+
+# NOTE: all of the following is copy-pasted from `/WORKSPACE` and reduced
+# to the necessary to make this repository work compatibly in isolation.
+
+# TODO: remove dependency on cumulative repository to be sure there is no
+# hidden entanglement left. use individual toolchains instead.
+local_repository(
+    name = "io_tweag_rules_nixpkgs",
+    path = "..",
+)
+
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+load("@io_tweag_rules_nixpkgs//nixpkgs:repositories.bzl", "rules_nixpkgs_dependencies")
+
+rules_nixpkgs_dependencies()
+
+load("//:nixpkgs.bzl", "nixpkgs_git_repository", "nixpkgs_local_repository")
+load("@rules_nixpkgs_cc//:cc.bzl", "nixpkgs_cc_configure")
+load("@rules_nixpkgs_java//:java.bzl", "nixpkgs_java_configure")
+
+http_archive(
+    name = "io_bazel_stardoc",
+    sha256 = "6d07d18c15abb0f6d393adbd6075cd661a2219faab56a9517741f0fc755f6f3c",
+    strip_prefix = "stardoc-0.4.0",
+    urls = [
+        "https://mirror.bazel.build/github.com/bazelbuild/stardoc/archive/0.4.0.tar.gz",
+        "https://github.com/bazelbuild/stardoc/archive/0.4.0.tar.gz",
+    ],
+)
+
+load("@io_bazel_stardoc//:setup.bzl", "stardoc_repositories")
+
+stardoc_repositories()
+
+nixpkgs_git_repository(
+    name = "remote_nixpkgs",
+    remote = "https://github.com/NixOS/nixpkgs",
+    revision = "21.11",
+    sha256 = "c77bb41cf5dd82f4718fa789d49363f512bb6fa6bc25f8d60902fe2d698ed7cc",
+)
+
+nixpkgs_local_repository(
+    name = "nixpkgs",
+    nix_file = "@io_tweag_rules_nixpkgs//:nixpkgs.nix",
+    nix_file_deps = ["@io_tweag_rules_nixpkgs//:nixpkgs.json"],
+)
+
+nixpkgs_cc_configure(
+    name = "nixpkgs_config_cc",
+    repository = "@remote_nixpkgs",
+)
+
+load("@rules_java//java:repositories.bzl", "rules_java_dependencies")
+
+rules_java_dependencies()
+
+nixpkgs_java_configure(
+    attribute_path = "jdk8.home",
+    repository = "@remote_nixpkgs",
+)