fix!: restrict graalvm tag to the root module

Unifying toolchain definitions across transitive modules is hard. By
restricting the usage of this tag to the root module, we can come up
with better API without breaking potential adopters in the BCR.

Signed-off-by: Fabian Meumertzheim <fabian@meumertzhe.im>

extensions.bzl

@@ -13,6 +13,8 @@ def _gvm_impl(mctx):
  for mod in mctx.modules:
  # gather gvm toolchain info
  for gvm in mod.tags.graalvm:
+ if not mod.is_root:
+ fail("graalvm tag is only allowed in the root module, use component tag instead")
  selected = gvm
  if len(gvm.components) > 0:
  all_components += [i for i in gvm.components if not i in all_components]