Android testFixtures cause DuplicateRelativeFileException in 0.10.0

In version 0.10.0

Enabling testFixtures on a module with
```
android {
testFixtures {
enable = true
}
}
```

will result in .proto files in the main sourceSet to be processed twice, once when compiling main and once when compiling the testFixture.

Depending on both the module and the testFixture (such as for tests) will then fail to merge resources since there are duplicate .proto resource files as well as duplicate generated sources.

Start with the Android testFixtures configuration shown in the issue and reproduce the failure in version 0.10.0. Trace how .proto files from the main sourceSet are processed for both main and testFixture, then verify that depending on both modules no longer produces duplicate resources or generated sources.

Written by the indexing model from the issue text.