Rename BUILD file to BUILD.bazel

Using standard naming to be consistent with the rest of the bp2build workspace.

Test: b build //build/core/...
Change-Id: If81a8fb82ccd3f63d0961f8a4a37ca8be7aa4430
diff --git a/core/BUILD b/core/BUILD.bazel
similarity index 100%
rename from core/BUILD
rename to core/BUILD.bazel