[CDTOOL-1257] Move healthcheck command under service

[rcaril]

Change summary

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?

 make test TEST_ARGS="-run TestHealthCheckCreate ./pkg/commands/service/healthcheck"
ok      github.com/fastly/cli/pkg/commands/service/healthcheck  1.048s
 make test TEST_ARGS="-run TestHealthCheckList ./pkg/commands/service/healthcheck"
ok      github.com/fastly/cli/pkg/commands/service/healthcheck  1.067s
make test TEST_ARGS="-run TestHealthCheckDescribe ./pkg/commands/service/healthcheck"
ok      github.com/fastly/cli/pkg/commands/service/healthcheck  1.052s
make test TEST_ARGS="-run TestHealthCheckUpdate ./pkg/commands/service/healthcheck"
ok      github.com/fastly/cli/pkg/commands/service/healthcheck  1.064s
make test TEST_ARGS="-run TestHealthChecDelete ./pkg/commands/service/healthcheck"
ok      github.com/fastly/cli/pkg/commands/service/healthcheck  1.023s [no tests to run]

Changes to Core Features:

• Have you written new tests for your core changes, as applicable?
• Have you successfully run tests with your changes locally?

User Impact

None as alias' have been implemented.