Exclude BUILD files from flit builds

[hartikainen]

This should fix #1429. Here's what I get from running flit build on main and the latest commit:

$ diff -ruN ./dist-main/orbax_checkpoint-0.11.0 ./dist-e6f8019a9f39/orbax_checkpoint-0.11.0
diff --color -ruN ./dist-main/orbax_checkpoint-0.11.0/orbax/checkpoint/_src/path/BUILD ./dist-e6f8019a9f39/orbax_checkpoint-0.11.0/orbax/checkpoint/_src/path/BUILD
--- ./dist-main/orbax_checkpoint-0.11.0/orbax/checkpoint/_src/path/BUILD        2024-12-25 21:28:52
+++ ./dist-e6f8019a9f39/orbax_checkpoint-0.11.0/orbax/checkpoint/_src/path/BUILD       1970-01-01 02:00:00
@@ -1,15 +0,0 @@
-package(
-    default_applicable_licenses = ["//:package_license"],
-    default_visibility = ["//visibility:public"],
-)
-
-py_library(
-    name = "utils",
-    srcs = ["utils.py"],
-    srcs_version = "PY3",
-)
-
-py_library(
-    name = "path",
-    srcs = ["__init__.py"],
-)
diff --color -ruN ./dist-main/orbax_checkpoint-0.11.0/orbax/checkpoint/_src/path/snapshot/BUILD ./dist-e6f8019a9f39/orbax_checkpoint-0.11.0/orbax/checkpoint/_src/path/snapshot/BUILD
--- ./dist-main/orbax_checkpoint-0.11.0/orbax/checkpoint/_src/path/snapshot/BUILD      2024-12-25 21:28:52
+++ ./dist-e6f8019a9f39/orbax_checkpoint-0.11.0/orbax/checkpoint/_src/path/snapshot/BUILD       1970-01-01 02:00:00
@@ -1,21 +0,0 @@
-package(
-    default_applicable_licenses = ["//:package_license"],
-    default_visibility = ["//visibility:public"],
-)
-
-py_library(
-    name = "snapshot",
-    srcs = ["snapshot.py"],
-    srcs_version = "PY3",
-    deps = ["//checkpoint/orbax/checkpoint/_src/path:utils"],
-)
-
-py_test(
-    name = "snapshot_test",
-    srcs = ["snapshot_test.py"],
-    srcs_version = "PY3",
-    deps = [
-        ":snapshot",
-        "//absl/testing:absltest",
-    ],
-)
diff --color -ruN ./dist-main/orbax_checkpoint-0.11.0/pyproject.toml ./dist-e6f8019a9f39/orbax_checkpoint-0.11.0/pyproject.toml
--- ./dist-main/orbax_checkpoint-0.11.0/pyproject.toml  2025-01-03 09:03:14
+++ ./dist-e6f8019a9f39/orbax_checkpoint-0.11.0/pyproject.toml  2025-01-03 09:04:18
@@ -41,6 +41,11 @@
 [tool.flit.module]
 name = "orbax.checkpoint"

+[tool.flit.sdist]
+exclude = [
+    '**/BUILD',
+]
+
 [project.urls]
 homepage = 'http://github.com/google/orbax'
 repository = 'http://github.com/google/orbax'