Review and correct internal/ vs pkg/ code location about singularity

[dtrudg]

Need to move from pkg -> internal/pkg:

• pkg/sypgp - needs a major refactor, see #79 , not expected to be stable.

Need to tidy up:

• pkg/ociruntime - only some constants used in e2e tests.

Need to remove:

• internal/pkg/util/exec - unused code

Candidates to move from internal/pkg -> pkg

These are stable enough, but might not be interesting / valuable to make available.

• internal/pkg/cgroups
• internal/pkg/fakeroot
• internal/pkg/signature
• internal/pkg/fs (helper.go)
• internal/pkg/util/machine
• internal/pkg/util/passwdfile
• internal/pkg/util/priv
• internal/pkg/util/signal

[dtrudg]

The required changes, from pkg -> internal/pkg, and removals, have been completed.

Consideration of exposing stable parts of internal/pkg can be tackled when time permits. Adding functionality with breakage is okay in a minor semver bump.