Add support for io/fs to load.Instances

[cueckoo]

Originally opened by @sdboyer in cuelang/cue#814

Is your feature request related to a problem? Please describe.
I have a set of CUE files that i'd like to have available for use with CUE's Go API inside a larger program. These files are knowable at compile time, and i'd like to use Go 1.16 new embed and io/fs to make them available within the program. However, i can't take advantage of load.Instances() with embedded files.

Describe the solution you'd like

The simplest thing i can imagine is:

package load

type Config {
    ...
    // DirFS represents the root of a hierarchical filesystem in which to run the
    // the build system's query tool that provides information about the packages.
    // It is an error to specify both Dir and DirFS.
    // If neither Dir nor DirFS are specified, the tool is run in the current directory.
    DirFS fs.ReadDirFS
}

i don't yet know the new io/fs package well enough to know exactly which composition of the fs interfaces would be sufficient for CUE's needs here. I'm also sure that the above simplistic formulation wouldn't work well if the provided FS is below the module root, so some relation between the above and Config.ModuleRoot probably needs to exist.

Describe alternatives you've considered
It's hard to imagine anything simpler here that allows use of io/fs. I could imagine something more elaborate that ends up as part of an approach to querying, but i can't think of a reason why that would be a reason not to support this simpler case.

[cueckoo]

Original reply by @verdverm in cuelang/cue#814 (comment)

related: cuelang/cue#607

Cue wants to align with the supported Go versions, i.e. since Go still supports 1.15 Cue does not want to force Go 1.16 on its users. io/fs is definitely a possibility down the road.

You can use io/fs in your own program, walk the filetree, and add them to load.Config.Overlay.

[cueckoo]

Original reply by @sdboyer in cuelang/cue#814 (comment)

Oh, nice! i'd missed Overlay. That'll do.