Improve caching of proto in GenerateProtoTask

This change will remove the OS from the input properties of the
`GenerateProtoTask`. The expectation is that same versions of `protoc`
will produce same outputs regardless of the OS on which it was run.

If a protoc `artifact` is defined with GAV parameters, the classifier
and extension will not be considered part of the input. An exception is
made for snapshots, which will always be considered uncacheable and be
re-run.

If a protoc `path` is defined, then the path will be exposed as a File
Provider with absolute path normalization since there is no good way to
tell the version of `protoc` without running `protoc --version`.

See google#457.

src/main/groovy/com/google/protobuf/gradle/GenerateProtoTask.groovy

@@ -29,6 +29,8 @@
  */
 package com.google.protobuf.gradle
 
+import org.gradle.api.tasks.InputFile
+
 import static java.nio.charset.StandardCharsets.US_ASCII
 
 import com.google.common.base.Preconditions
@@ -330,8 +332,38 @@ public abstract class GenerateProtoTask extends DefaultTask {
   @Internal("Input captured by getAlternativePaths(), this is used to query alternative path by locator name.")
   abstract MapProperty<String, FileCollection> getLocatorToAlternativePathsMapping()
 
-  @InputFiles
-  @PathSensitive(PathSensitivity.NONE)
+  @Input
+  @Optional
+  Provider<String> getProtocArtifact() {
+    // Ideally, this value could be mapped from the protocLocator property, but Provider#map cannot return null in
+    // Gradle < 6.2. See https://github.com/gradle/gradle/issues/11979
+    return providerFactory.provider {
+      def protocArtifact = protocLocator.get().artifact
+      if (protocArtifact) {
+        def (groupId, artifact, version) = ToolsLocator.artifactParts(protocArtifact)
+        outputs.doNotCacheIf("protoc is a SNAPSHOT version", {
+          version.endsWith("-SNAPSHOT")
+        })
+        "$groupId:$artifact:$version".toString()
+      } else {
+        null
+      }
+    }
+  }
+
+  @InputFile
+  @PathSensitive(PathSensitivity.ABSOLUTE)
+  @Optional
+  Provider<File> getProtocPath() {
+    // Ideally, this value could be mapped from the protocLocator property, but Provider#map cannot return null in
+    // Gradle < 6.2. See https://github.com/gradle/gradle/issues/11979
+    return providerFactory.provider {
+      def path = getProtocLocator().get().path
+      path ? new File(path) : null
+    }
+  }
+
+  @Internal
   ConfigurableFileCollection getAlternativePaths() {
     return objectFactory.fileCollection().from(getLocatorToAlternativePathsMapping().get().values())
   }