frontend: add required paths to LLB and use it with --parents

[jsternberg]

This adds an additional RequiredPaths that is primarily intended for
use with COPY --parents. This parameter specifies expected directories
or files that should exist when performing the checksum. A not found
error will be produced if one of these paths is missing.

This fixes an issue with COPY --parents where a non existent directory
that was intended to be copied would be ignored.

Fixes #4900.

[jsternberg]

Need to fix the tests but I think this method of fixing it is probably the easiest.

[tonistiigi]

Is there a reason why this needs to take paths, instead of just being boolean? If it is boolean then I guess if no paths match then it returns a constant "zero checksum" that would be possible to check on the caller side.

[jsternberg]

I'm a little fuzzy on why I went this direction exactly compared to the boolean, but I think there were two reasons.

1. The error message that gave the path was really difficult to figure out without putting the paths explicitly.
2. Wildcards complicated the boolean method because the correct prefix wasn't necessarily trivial to determine from the checksum path.

Adding the required paths as an explicit argument took away the guesswork involved with this. The main problem is we needed information both about the pivot point and the non-wildcard section of the pattern to determine which path we needed so it wasn't as simple as just saying "if things are empty it's invalid" because the directory being empty could still be valid.