Add is_root struct field to bazel_module

[fmeum]

This allows module extensions to check whether a given module is the
root module, which is necessary to implement the analogues of
--check_direct_dependencies and archive_override/git_override for
dependencies managed by extensions.

[fmeum]

@meteorcloudy @Wyverald This seemed straightforward enough for me to go for a PR right away. If you want to discuss the approach more generally, I could create a feature request instead.

[meteorcloudy]

@Wyverald I thought we already have a way to tell which one is the root module. IIUC, the modules are listed in BFS order, so the first module must be root module, is that right?

[fmeum]

@Wyverald I thought we already have a way to tell which one is the root module. IIUC, the modules are listed in BFS order, so the first module must be root module, is that right?

The modules are listed in BFS order, but seem to be filtered to include only those that use the given extension, meaning that the root module may not even appear in the list:

bazel/src/main/java/com/google/devtools/build/lib/bazel/bzlmod/SingleExtensionUsagesFunction.java

Line 57 in 0152338

// Filter abridged modules down to only those that actually used this extension.

[meteorcloudy]

Oh, I see! Then this change looks good to me, but waiting for @Wyverald to confirm!

[fmeum]

@bazel-io flag

[meteorcloudy]

@bazel-io fork 5.3.0

[ckolli5]

@bazel-io fork 5.3.0