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