archive/zip: (*File).Open is inconsistent with (*Reader).Open for directory entries

[bcmills]

What version of Go are you using (go version)?

https://pkg.go.dev/archive/zip@go1.20rc3

Does this issue reproduce with the latest release?

Yes

What did you do?

(https://go.dev/play/p/NFjouX0xrQk?v=gotip)

1. Create a zip archive containing a directory entry.
2. Open a zip.Reader for that archive.
3. Open the zip.File for the directory entry using (*zip.Reader).Open and Read it.
4. Open the zip.File for the directory by calling (*zip.File).Open and Read it.

What did you expect to see?

The same contents (and same error code) regardless of how the directory entry was opened.

What did you see instead?

The reader opened via (*zip.Reader).Open returns a non-nil error like read dir: is a directory.
The reader opened via (*zip.File).Open returns 0, io.EOF.

(attn @dsnet @bradfitz; CC @neild)

[bcmills]

(Found while investigating #53448.)

[bcmills]

I'm not sure whether this can be addressed without breaking something.

(Arguably the lack of error return from the Read call from (*zip.File).Open is a bug, but is the bug too entrenched at this point to fix?)

[rsc]

It's a clear bug for the zip.File.Open result to return io.EOF from a read on a directory. We should fix that. This is the kind of thing that can come with a GODEBUG to ease any transition (#56986).

[rsc]

Note that we should update HashZip (#53448) to avoid the error before or at the same time as making this change, so that module archives that work today keep hashing to the same value rather than failing to hash at all.