Deprecate pkg/user package?

[tri-adam]

The user package was originally added to support the SingularityCE Roadmap, as well as support reuse in packages such as Anchore Stereoscope. With SingularityCE having its own (arguably more advanced) FUSE code, and having gone a different direction with Stereoscope (anchore/stereoscope#125), it may make sense to deprecate this package (and remove it in the next major version.)

As additional data: pkg.go.dev is not currently showing any known importers aside from github.com/sylabs/singularity/v4 (ref: https://pkg.go.dev/github.com/sylabs/sif/v2/pkg/user?tab=importedby).

[preminger]

If & when sylabs/singularity#2199 is merged, the aforementioned imports from github.com/sylabs/singularity/v4 will have been removed.