Snapshot output directory should be a task input not output

[jrodbx]

We want Gradle's task invalidation logic to rerun tests when the snapshot output directory is changed, which was the motivation behind #206. However, by adding it as an output property, Gradle 8.0 now throws the following error:

A problem was found with the configuration of task ':sample:testReleaseUnitTest' (type 'AndroidUnitTest').
  - Gradle detected a problem with the following location: '/home/runner/work/paparazzi/paparazzi/sample/src/test/snapshots'.
    
    Reason: Task ':sample:spotlessKotlin' uses this output of task ':sample:testReleaseUnitTest' without declaring an explicit or implicit dependency. This can lead to incorrect results being produced, depending on what order the tasks are executed.
    
    Possible solutions:
      1. Declare task ':sample:testReleaseUnitTest' as an input of ':sample:spotlessKotlin'.
      2. Declare an explicit dependency on ':sample:testReleaseUnitTest' from ':sample:spotlessKotlin' using Task#dependsOn.
      3. Declare an explicit dependency on ':sample:testReleaseUnitTest' from ':sample:spotlessKotlin' using Task#mustRunAfter.
    
    Please refer to https://docs.gradle.org/8.1.1/userguide/validation_problems.html#implicit_dependency for more details about this problem.

Spotless doesn't care about snapshots, and output side effects of a task don't necessarily == task outputs, so this fixes both.

Side note: This change specifically uses inputs.files(...), instead of inputs.dir(...), since the latter requires the folder to exist per Gradle conventions: https://docs.gradle.org/7.6.1/userguide/validation_problems.html#input_file_does_not_exist

[TWiStErRob]

Add "side note" as a comment here to prevent inadvertent future "optimizations"?

[TWiStErRob]

I have a feeling this ambiguity comes from the fact that the same task can run in record and verify modes. In record mode the output is snapshots, and in verify mode snapshots are input and the output is reports. Does this mean that adding snapshotOutputDir as input/output should be based on the isRecordRun/isVerifyRun properties? (e.g. something like isRecordRun.map { if (it) snapshotOutputDir else "dummyNonExistentFolder" })?