feat(toolchain, pip.parse): introduce a new '_host' toolchain repo (#1644)

This is for passing it in repository_rules and relies on the canonical
label representation introduced in bazel 6.0 and symlink support (needs
to be present on Windows) to work.

This allows the users to not need to `load` the interpreter label from
a `.bzl` file but instead specify the label in the form of
`@<mytoolchainrepo>_host//:python`.

In order to make it work robustly on Windows, we do
`repository_ctx.path(python_interpreter_label).realpath` to get the
actual
path of the Python interpreter on both, bzlmod and non-bzlmod setups on
Windows within the `whl_library` repository rule. 

Work towards #1643.

examples/multi_python_versions/WORKSPACE

diff --git a/examples/multi_python_versions/WORKSPACE b/examples/multi_python_versions/WORKSPACE
index 35855ca..f3a69ce 100644
--- a/examples/multi_python_versions/WORKSPACE
+++ b/examples/multi_python_versions/WORKSPACE
@@ -28,19 +28,15 @@
 )
 
 load("@python//:pip.bzl", "multi_pip_parse")
-load("@python//3.10:defs.bzl", interpreter_3_10 = "interpreter")
-load("@python//3.11:defs.bzl", interpreter_3_11 = "interpreter")
-load("@python//3.8:defs.bzl", interpreter_3_8 = "interpreter")
-load("@python//3.9:defs.bzl", interpreter_3_9 = "interpreter")
 
 multi_pip_parse(
     name = "pypi",
     default_version = default_python_version,
     python_interpreter_target = {
-        "3.10": interpreter_3_10,
-        "3.11": interpreter_3_11,
-        "3.8": interpreter_3_8,
-        "3.9": interpreter_3_9,
+        "3.10": "@python_3_10_host//:python",
+        "3.11": "@python_3_11_host//:python",
+        "3.8": "@python_3_8_host//:python",
+        "3.9": "@python_3_9_host//:python",
     },
     requirements_lock = {
         "3.10": "//requirements:requirements_lock_3_10.txt",