bootclasspath_fragment must only depend on source contents

This change ensures that bootclasspath_fragment only depends on source
modules and prebuilt_bootclasspath_fragment only depends on prebuilt
modules.

It does that in two ways:
1. It adds the dependencies in ComponentDepsMutator method which is
   called before any renaming of prebuilts is done which makes it very
   easy to add a dependency directly onto either the source or prebuilt
   as required.
2. It uses a tag which prevents dependencies on a source module from
   being replaced with a dependency on a prebuilt module which ensures
   that a dependency on the source modules is not replaced with a
   dependency on a prebuilt module.

Bug: 177892522
Test: m nothing
Change-Id: Ibcfae39083afbc07fcf729ead3ed5f5d020845bf

apex/boot_image_test.go

@@ -216,6 +216,22 @@ func TestBootclasspathFragmentInArtApex(t *testing.T) {
 			],
 		}
 
+		java_import {
+			name: "foo",
+			jars: ["foo.jar"],
+			apex_available: [
+				"com.android.art",
+			],
+		}
+
+		java_import {
+			name: "bar",
+			jars: ["bar.jar"],
+			apex_available: [
+				"com.android.art",
+			],
+		}
+
 		// Make sure that a preferred prebuilt doesn't affect the apex.
 		prebuilt_boot_image {
 			name: "mybootclasspathfragment",