Bump Bazel to 7.0 (#2006)

Major change: Bump Bazel to the new LTS at 7.X

* Included a default Python toolchain 3.10

  There is a problem at the moment running lit using Python311 where it
  cannot find the module.

  Our CI system already enforces Python310 to get around it but we should
  set the toolchain in Bazel itself to avoid this problem.
  I had already filed llvm/llvm-project#75963

* I moved skylib to MODULE.bazel to test the MODULE portion of Bazel is
working. A remaining action item is to move LLVM itself to MODULE.bazel
which would preclude needing to define skylib since its a dependency of
LLVM.

* Workspace names are no longer named in Bazel 7+
  
  I had to change stablehlo to _main to fix the runfiles directory.
  Please see bazelbuild/bazel#18128

We should consider using
https://github.com/bazelbuild/rules_python/blob/main/python/runfiles/runfiles.py#L262
instead to get the runfiles directory for future proofing.

Fixes #1878

---------

Co-authored-by: mlevesquedion <[email protected]>

.bazelversion

@@ -1,4 +1,4 @@
-6.1.0
+7.0.2
 # Copyright 2023 The StableHLO Authors. All Rights Reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");

MODULE.bazel

@@ -0,0 +1,19 @@
+###############################################################################
+# Bazel now uses Bzlmod by default to manage external dependencies.
+# Please consider migrating your external dependencies from WORKSPACE to MODULE.bazel.
+#
+# For more details, please check https://github.com/bazelbuild/bazel/issues/18958
+###############################################################################
+bazel_dep(name = "bazel_skylib", version = "1.3.0")
+bazel_dep(name = "rules_python", version = "0.30.0")
+
+python = use_extension("@rules_python//python/extensions:python.bzl", "python")
+
+# There is a problem at the moment running lit using Python 3.11
+# Our CI system enforces Python 3.10 to get around it but we should set the toolchain
+# in Bazel itself to avoid this problem.
+# https://github.com/llvm/llvm-project/issues/75963
+python.toolchain(
+    is_default = True,
+    python_version = "3.10",
+)