Add the concept of "available peripherals" to IComputerAccess

This provides a mechanism for peripherals to see what else a computer is
connected to - and then interact with those peripherals.

We also add the ability to query what block or tile a peripheral
targets. This allows one to interact with the original block of adjacent
peripherals instead.

src/main/java/dan200/computercraft/api/peripheral/IComputerAccess.java

@@ -13,6 +13,8 @@ import net.minecraft.world.World;
 
 import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
+import java.util.Collections;
+import java.util.Map;
 
 /**
  * The interface passed to peripherals by computers or turtles, providing methods
@@ -154,4 +156,33 @@ public interface IComputerAccess
      */
     @Nonnull
     String getAttachmentName();
+
+    /**
+     * Get a set of peripherals that this computer access can "see", along with their attachment name.
+     *
+     * This may include other peripherals on the wired network or peripherals on other sides of the computer.
+     *
+     * @return All reachable peripherals
+     * @see #getAttachmentName()
+     * @see #getAvailablePeripheral(String)
+     */
+    @Nonnull
+    default Map<String, IPeripheral> getAvailablePeripherals()
+    {
+        return Collections.emptyMap();
+    }
+
+    /**
+     * Get a reachable peripheral with the given attachement name. This is a equivalent to
+     * {@link #getAvailablePeripherals()}{@code .get(name)}, though may be more performant.
+     *
+     * @param name The peripheral's attached name
+     * @return The reachable peripheral, or {@code null} if none can be found.
+     * @see #getAvailablePeripherals()
+     */
+    @Nullable
+    default IPeripheral getAvailablePeripheral( @Nonnull String name )
+    {
+        return null;
+    }
 }

src/main/java/dan200/computercraft/api/peripheral/IPeripheral.java

@@ -114,6 +114,18 @@ public interface IPeripheral
     {
     }
 
+    /**
+     * Get the object that this peripheral provides methods for. This will generally be the tile entity
+     * or block, but may be an inventory, entity, etc...
+     *
+     * @return The object this peripheral targets
+     */
+    @Nonnull
+    default Object getTarget()
+    {
+        return this;
+    }
+
     /**
      * Determine whether this peripheral is equivalent to another one.
      *