Partial rollback of bazelbuild@934c4fd to restore package

The scala_test rules reference the lcov merger as @bazel_tools//tools/test/CoverageOutputGenerator/java/com/google/devtools/coverageoutputgenerator:Main, so restore the package and add an alias to avoid breaking them.

This change can be reverted once the downstream rules are fixed.

Fixes bazelbuild#10425.

```
$ /tmp/bazel test --flaky_test_attempts=3 --build_tests_only --experimental_repository_cache_hardlinks --disk_cache= --sandbox_tmpfs_path=/tmp --test_env=HOME //...
INFO: Analyzed 2 targets (0 packages loaded, 0 targets configured).
INFO: Found 2 test targets...
INFO: From Building external/com_google_protobuf/libprotobuf_java.jar (122 source files, 1 source jar):
warning: -parameters is not supported for target value 1.7. Use 1.8 or later.
INFO: Elapsed time: 85.014s, Critical Path: 42.07s
INFO: 273 processes: 260 darwin-sandbox, 13 worker.
INFO: Build completed successfully, 274 total actions
//:test_test_suite_src_test_scala_hello_ConfigurationSpec.scala          PASSED in 1.9s
//:test_test_suite_src_test_scala_hello_EndpointSpec.scala               PASSED in 2.6s

INFO: Build completed successfully, 274 total actions
```
PiperOrigin-RevId: 285928101

tools/BUILD

@@ -61,6 +61,7 @@ filegroup(
         "//tools/python:embedded_tools",
         "//tools/runfiles:embedded_tools",
         "//tools/test:embedded_tools",
+        "//tools/test/CoverageOutputGenerator/java/com/google/devtools/coverageoutputgenerator:embedded_tools",
         "//tools/osx/crosstool:srcs",
         "//tools/osx:srcs",
         "//tools/sh:embedded_tools",

tools/test/CoverageOutputGenerator/java/com/google/devtools/coverageoutputgenerator/BUILD

@@ -172,3 +172,11 @@ filegroup(
     srcs = glob(["**"]),
     visibility = ["//visibility:public"],
 )
+
+filegroup(
+    name = "embedded_tools",
+    srcs = [
+        "BUILD.tools",
+    ],
+    visibility = ["//visibility:public"],
+)

tools/test/CoverageOutputGenerator/java/com/google/devtools/coverageoutputgenerator/BUILD.tools

@@ -0,0 +1,6 @@
+package(default_visibility = ["//visibility:public"])
+
+alias(
+    name = "Main",
+    actual = "//tools/test:lcov_merger",
+)