Skip `contents` validation of source bootclasspath fragments

... if prebuilts are selected.

This is a special case of b/361771739 where a apex starts contributing a
jar to the BCP. `disableSourceApexVariant` uses
`PrebuiltSelectionInfoProvider` and this gets populated via the
following dep chain
```
source --> prebuilt --> all_apex_contributions
```

This runs into issues if a prebuilt bootclasspath_fragment does not
exist.

This CL creates a dependency edge between the source module and
all_apex_contributions singleton module so that
`PrebuiltSelectionInfoProvider` is available for
`disableSourceApexVariant`

Bug: 361771739
Test: lunch cf_x86_64-next-userdebug
Test: m nothing with ag/28851886
Change-Id: I92d88199a27579d3c9879d40fd354653370efcce
diff --git a/apex/bootclasspath_fragment_test.go b/apex/bootclasspath_fragment_test.go
index 25131ee..fd2f3c5 100644
--- a/apex/bootclasspath_fragment_test.go
+++ b/apex/bootclasspath_fragment_test.go
@@ -833,6 +833,7 @@
 	`)
 
 	java.CheckModuleDependencies(t, result.TestContext, "mybootclasspathfragment", "android_common_apex10000", []string{
+		"all_apex_contributions",
 		"art-bootclasspath-fragment",
 		"bar",
 		"dex2oatd",
@@ -1003,6 +1004,7 @@
 	`)
 
 	java.CheckModuleDependencies(t, result.TestContext, "mybootclasspathfragment", "android_common_apex10000", []string{
+		"all_apex_contributions",
 		"android-non-updatable.stubs",
 		"android-non-updatable.stubs.module_lib",
 		"android-non-updatable.stubs.system",
@@ -1174,6 +1176,7 @@
 	`)
 
 	java.CheckModuleDependencies(t, result.TestContext, "mybootclasspathfragment", "android_common_apex10000", []string{
+		"all_apex_contributions",
 		"android-non-updatable.stubs",
 		"android-non-updatable.stubs.system",
 		"android-non-updatable.stubs.test",
@@ -1326,6 +1329,7 @@
 	`)
 
 	java.CheckModuleDependencies(t, result.TestContext, "mybootclasspathfragment", "android_common_apex10000", []string{
+		"all_apex_contributions",
 		"art-bootclasspath-fragment",
 		"bar",
 		"dex2oatd",