build: don't stat directories that we don't plan to upload

[nicks]

Hello @milas, @landism,

Please review the following commits I made in branch nicks/walk:

c1a8373 (2022-05-04 19:06:56 -0400)
build: don't stat directories that we don't plan to upload
creates some temporary patches on buildkit until we can upstream them

fixes #5745

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[nicks]

I'm hoping the upstream patches will be accepted, because they're pretty minimal/non-invasive (which you can see in the vendor diff). But figure we'll have to patch anyway because we're on an older version of buildkit.

[milas]

But figure we'll have to patch anyway because we're on an older version of buildkit.

FWIW if the fork(s) become onerous to maintain, we could probably get most of the way there by using the existing Excludes field on SyncedDir. We'd have to populate from v1alpha1.IgnoreDef, which would require some care to avoid it kicking right back down the slow path, so it's not ideal for a variety of reasons

[nicks]

ya, it's tricky because ExcludePatterns MUST be relative to the context, so would need to do a fair bit of re-formatting