Review and correct internal/ vs pkg/ code location

[dtrudg]

Type of issue
technical debt

Description of issue
Various portions of code in public pkg/ areas are not likely to be stable over time at present. This should be worked out so that it is possible to move toward the expectation that 3rd parties can use pkg/ functions with some stability, and as a stepping stone toward Go module semantic versioning conformance.

[dtrudg]

Need to move from pkg -> internal/pkg:

• pkg/sypgp - needs a major refactor, see Overhaul key command #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.