Make ImmutableMap Stream collect()ion nicer (#654)

This adds an entriesToImmutableMap() collector that can be used in place of
toImmutableMap(Map.Entry::getkey, Map.Entry::getValue()).

It also fixes up some existing calls that use toImmutableMap() when terser
alternatives exist.

buildSrc/src/main/java/google/registry/gradle/plugin/CoverPageGenerator.java

@@ -29,6 +29,7 @@ import google.registry.gradle.plugin.ProjectData.TaskData;
 import java.io.File;
 import java.nio.file.Path;
 import java.nio.file.Paths;
+import java.util.Map;
 import java.util.function.Supplier;
 import java.util.stream.Collectors;
 
@@ -162,7 +163,7 @@ final class CoverPageGenerator {
             task.reports().entrySet().stream()
                 .collect(
                     toImmutableMap(
-                        entry -> entry.getKey(),
+                        Map.Entry::getKey,
                         entry ->
                             entry.getValue().files().isEmpty()
                                 ? ""

buildSrc/src/main/java/google/registry/gradle/plugin/GcsPluginUtils.java

@@ -168,7 +168,7 @@ final class GcsPluginUtils {
             .collect(
                 toImmutableMap(
                     file -> rootDir.relativize(toNormalizedPath(file)),
-                    file -> toByteArraySupplier(file)));
+                    GcsPluginUtils::toByteArraySupplier));
 
     if (files.isEmpty()) {
       // The directory exists, but is empty. Return empty FilesWithEntryPoint