CDTOOL-1260: Introduce 'service purge' command and 'purge' alias.

[kpfleming]

Change summary

This PR adds the 'service purge' command as a replacement for the 'purge' command. The 'purge' command is still available as an alias, but will issue a deprecation warning when it is used.

This involved a number of changes:

1. The existing 'pkg/commands/purge' package content was moved to 'pkg/commands/service/purge', and modified to work as expected in that location.
2. A new 'pkg/commands/alias' package was added to hold alias-only packages.
3. An alias was added in 'pkg/commands/alias/purge'. This alias uses struct embedding to avoid duplicating any of the code from the `pkg/commands/service/purge' package. The alias command is 'unlisted' and issues a deprecation warning when it is used.

This PR also fixes the make fmt target in the Makefile to work properly.

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

• Does your submission pass tests?

User Impact

New deprecation warning for the purge command.

[kpfleming]

Since this is a new source file Revive complains about this structure name, but I've suppressed the warning since the code is not new, it was just relocated, and this naming is consistent with other existing commands.

[anthony-gomez-fastly]

LGTM

[rcaril]

Looks solid!

[philippschulte]

This looks great! Thanks!