Fix incomplete incremental inputs
Fix incomplete incremental inputs

When you cancel an action, some of the incremental outputs may be
produced while some may not. Since Swift 5.6 outputs from swift are not
always created if nothing has changed, which lead to subsequent actions
failing. Now before copying over any incremental inputs, we first
validate that all the expected files exist, otherwise we don't copy any
to force swift to recreate everything.

Fixes https://github.com/bazelbuild/rules_swift/issues/819

bazelbuild/rules_swift,bazelbuild/rules_swift