[roll] Roll fuchsia [bazel] Fix `no_sdk = true` incrementality failures.

This fixes flaky incremental build failures introduced by using
`no_sdk = true` in GN `bazel_action()` target, by *completely*
refactoring the way the Bazel workspace is setup when
`--config=no_sdk` is used.

In a nutshell:

- Both `fx bazel` and `bazel_action.py` will change the workspace
  configuration, based on the presence of `--config=no_sdk` when
  invoking Bazel.

- When `--config=no_sdk` is not present, root Bazel files like
  `WORKSPACE.bazel` or `.bazelrc` will point to versions of these
  file that contain the full set of Fuchsia-related definitions,
  including the ones that define the `@fuchsia_sdk`.

- When `--config=no_sdk` is present, the same files will point
  to reduced versions that never define `@fuchsia_sdk` or other
  IDK/SDK-related repositories or settings.

By doing this, the content of $OUTPUT_BASE/external/fuchsia_sdk
is preserved when switching between fuchsia and non-sdk workspace
variants (which can happen in a single `fx build` invocation)
and the incrementality issues disappear.

This is a temporary work-around until the @fuchsia_in_tree_idk
can be generated at `fx gen` time directly, which will allow
removing this scheme.

- toplevel.WORKSPACE.bazel, toplevel.BUILD.bazel, template.bazelrc,
  template.platform_mappings: Move Fuchsia-specific definitions
  after a special marker line starting with "### FUCHSIA_SDK_CUTOFF"
  which is used to generate the no-sdk variants of these files
  by omitting that line and everything that follows it.

- workspace_utils.py: Add RootBazelFilesVariantsGenerator class
  to handle the generation of the multiple variants of root
  Bazel files in the workspace.

- bazel_utils.sh: Detect --config=no_sdk on the command-line
  and adjust the bazel_root_files symlink accordingly
  before launching Bazel.

- bazel_action.py: Do the same from Python.

- build/bazel/README.md: Fix a few obsolete paragraphs and
  add a new section documenting the behavior.

- build/bazel/examples/hello_no_sdk: Add a new bazel_action()
  target that sets `no_sdk = true`, and add it to the set of
  Bazel related tests to exercise switching between fuchsia
  and no_sdk bazel actions in CQ.

Original-Fixed: 401120294
Original-Bug: 322881092

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1253187
Original-Revision: a560caaa61a7ece94f19013869d06a41161f7eb9
GitOrigin-RevId: e817fda9d3de38937d4b12adaef3775e2c80e21e
Change-Id: I777866c781471cafbb5877847d5a84ae6ff0d70a