Fix hashing of directories

[rcannood]

Files.walkFileTree does not guarantee any specific order for iterating over files within a directory:

Considerations When Creating a FileVisitor

A file tree is walked depth first, but you cannot make any assumptions about the iteration order that subdirectories are visited.

-- source

This causes the sha of HashBuilder.hashDirSha256 to be different in certain situations (#6112).

This PR fixes the issue by storing all of the (path, sha256) in an array list, sorting by path, and then passing them to the hasher.

Notes:

• The code in preVisitDirectory seems like it would cause a null pointer exception if the condition (base != null) is false -- so maybe base is always not null? What should happen in this code chuck if base does happen to be null?
• It's hard to add a unit test for this, since the issue only arises in very specific situations.

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