Declare a "scalac" exec_group so per-action cpu sizing can be set

[gergelyfabian]

Adds an empty exec_group("scalac") to every scala rule that compiles Scala sources (scala_library, scala_library_for_plugin_bootstrapping, scala_macro_library, scala_binary, scala_test, scala_junit_test, scala_repl) and wires the Scalac action to it via
exec_group = "scalac" in compile_scala. Also declares the same exec_group on the two aspects that call compile_scala (scrooge_scala_aspect, scala_proto_aspect) — aspects do not inherit exec_groups from the rule they attach to, so they need their own declaration.

The group is intentionally empty (no toolchains / no exec_compatible_with) — its sole purpose is to be a target for the <group>.<key> syntax on exec_properties. This lets a user set exec_properties = {"scalac.<key>": "<value>"} on a target to configure the compile action independently of the target's other actions (test execution, jar packaging, etc.). Without a declared exec_group, Bazel rejects the prefixed key at analysis time.

The action's toolchain resolution and exec platform selection are unchanged: no toolchains or constraints are attached to the group, so the rule's default toolchains still apply to the Scalac action.

scala_proto_library and scrooge_scala_library intentionally do not declare the exec_group on the top-level rule: Bazel does not propagate target-level exec_properties to actions created by attached aspects, so declaring it there would silently drop the user's config. The analysis-time error surfaces the limitation; users needing per-action config for these rules should set exec_properties at the platform level.

Regression coverage:

• //test:ScalacExecGroup* targets set a scalac.* exec_property on each of the seven rules that support the feature. Removing the rule's exec_group declaration fails analysis on these targets under bazel build test/....
• test/shell/test_scalac_exec_group.sh uses bazel aquery to verify the property reaches the Scalac action's ExecutionInfo on every regression target, catching removal of exec_group = "scalac" from ctx.actions.run in compile_scala (which would silently reroute the property to the default exec group without failing analysis).
• //test_expect_failure/scalac_exec_group_on_aspect_rule/... exercises the intentional gap on scala_proto_library and scrooge_scala_library; the same shell test asserts both fail analysis with the expected "non-existent exec groups: scalac" message, so the docs' claim stays honest if Bazel changes behavior or someone adds the exec_group to those rules.

Documents the feature and the aspect-propagation limitation under docs/scalac_exec_group.md.