Split up actions java library target.

Notably, this separates Artifact and related types so that they don't have to be tied in with the rest of the actions library.

Pulled some nested classes to the top level to achieve this and did some other drive-by cleanups, but otherwise just import statements and BUILD files.

RELNOTES: None.
PiperOrigin-RevId: 322699250
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/ActionTemplateExpansionValue.java b/src/main/java/com/google/devtools/build/lib/skyframe/ActionTemplateExpansionValue.java
index ae4e53a..2a97f01 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/ActionTemplateExpansionValue.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/ActionTemplateExpansionValue.java
@@ -15,6 +15,7 @@
 
 import com.google.common.base.MoreObjects;
 import com.google.common.collect.Interner;
+import com.google.devtools.build.lib.actions.ActionLookupKey;
 import com.google.devtools.build.lib.actions.Actions.GeneratingActions;
 import com.google.devtools.build.lib.actions.BasicActionLookupValue;
 import com.google.devtools.build.lib.cmdline.Label;
@@ -36,7 +37,7 @@
 
   /** Key for {@link ActionTemplateExpansionValue} nodes. */
   @AutoCodec
-  public static final class ActionTemplateExpansionKey extends ActionLookupKey {
+  public static final class ActionTemplateExpansionKey implements ActionLookupKey {
     private static final Interner<ActionTemplateExpansionKey> interner =
         BlazeInterners.newWeakInterner();
 
@@ -70,7 +71,7 @@
 
     /**
      * Index of the action in question in the node keyed by {@link #getActionLookupKey}. Should be
-     * passed to {@link ActionLookupValue#getAction}.
+     * passed to {@link com.google.devtools.build.lib.actions.ActionLookupValue#getAction}.
      */
     int getActionIndex() {
       return actionIndex;