PackageUtil should retry check for whether file is already staged

The code to stage files retries the upload, but the existence (and size) check is attempted only once:

https://github.com/apache/beam/blob/dc3a664e3b4b4f63500954aae25dce0cb5f8b314/runners/google-cloud-dataflow-java/src/main/java/org/apache/beam/runners/dataflow/util/PackageUtil.java#L193

This causes post-commit tests to fail sometimes:

https://builds.apache.org/job/beam_PostCommit_Java_ValidatesRunner_Dataflow/org.apache.beam$beam-runners-google-cloud-dataflow-java/4632/testReport/junit/org.apache.beam.sdk.io/BoundedReadFromUnboundedSourceTest/testDedup/

https://builds.apache.org/job/beam_PostCommit_Java_ValidatesRunner_Dataflow/org.apache.beam$beam-runners-google-cloud-dataflow-java/4636/testReport/junit/org.apache.beam.sdk.transforms/ViewTest/testEmptyMultimapSideInput/

We should retry this check as well.

Imported from Jira [BEAM-3408](https://issues.apache.org/jira/browse/BEAM-3408). Original Jira may contain additional context.
Reported by: herohde.

Inspect runners/google-cloud-dataflow-java/src/main/java/org/apache/beam/runners/dataflow/util/PackageUtil.java around line 193, then compare the existing upload retry behavior with the one-time existence and size check. Run the named Dataflow post-commit tests, including BoundedReadFromUnboundedSourceTest/testDedup and ViewTest/testEmptyMultimapSideInput; done means the check retries and those intermittent failures no longer occur.

Written by the indexing model from the issue text.