Remove BaselineCoverageArtifactsProvider in favor of an output group.

The only slightly different thing here is that now, instead of using target.getConfiguration().isCodeCoverageEnabled() we use BuildRequest.Options.collectCodeCoverage, but the only place where this is not the same I can think of is InputFileCT, which does not have baseline coverage files anyway.

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MOS_MIGRATED_REVID=86682774
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/BuildView.java b/src/main/java/com/google/devtools/build/lib/analysis/BuildView.java
index 59adaf7..91a9256 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/BuildView.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/BuildView.java
@@ -40,6 +40,7 @@
 import com.google.devtools.build.lib.analysis.config.BuildConfiguration;
 import com.google.devtools.build.lib.analysis.config.BuildConfigurationCollection;
 import com.google.devtools.build.lib.analysis.config.ConfigMatchingProvider;
+import com.google.devtools.build.lib.collect.nestedset.NestedSet;
 import com.google.devtools.build.lib.collect.nestedset.NestedSetBuilder;
 import com.google.devtools.build.lib.collect.nestedset.Order;
 import com.google.devtools.build.lib.concurrent.ThreadSafety.ThreadCompatible;
@@ -767,17 +768,18 @@
         artifactsToBuild, parallelTests, exclusiveTests, topLevelOptions);
   }
 
-  private static ImmutableSet<Artifact> getBaselineCoverageArtifacts(
+  private static NestedSet<Artifact> getBaselineCoverageArtifacts(
       Collection<ConfiguredTarget> configuredTargets) {
-    Set<Artifact> baselineCoverageArtifacts = Sets.newHashSet();
+    NestedSetBuilder<Artifact> baselineCoverageArtifacts = NestedSetBuilder.stableOrder();
     for (ConfiguredTarget target : configuredTargets) {
-      BaselineCoverageArtifactsProvider provider =
-          target.getProvider(BaselineCoverageArtifactsProvider.class);
+      TopLevelArtifactProvider provider = target.getProvider(TopLevelArtifactProvider.class);
       if (provider != null) {
-        baselineCoverageArtifacts.addAll(provider.getBaselineCoverageArtifacts());
+        baselineCoverageArtifacts.addTransitive(provider.getOutputGroup(
+            TopLevelArtifactProvider.BASELINE_COVERAGE
+        ));
       }
     }
-    return ImmutableSet.copyOf(baselineCoverageArtifacts);
+    return baselineCoverageArtifacts.build();
   }
 
   private void addExtraActionsIfRequested(BuildView.Options viewOptions,