Fix ./pants help $goal showing the goal in "related subsystems"

[Eric-Arellano]

Before:

▶ ./pants help target-types

`target-types` goal options
---------------------------

List all the registered target types, including custom plugin types.

Config section: [target-types]

...
  --target-types-details=<target_type>
  PANTS_TARGET_TYPES_DETAILS
  details
      default: None
      current value: None
      List all of the target type's registered fields.


Related subsystems: target-types

After:

▶ ./pants help target-types

`target-types` goal options
---------------------------

List all the registered target types, including custom plugin types.

Config section: [target-types]

...

  --target-types-details=<target_type>
  PANTS_TARGET_TYPES_DETAILS
  details
      default: None
      current value: None
      List all of the target type's registered fields.

[ci skip-build-wheels]
[ci skip-rust]

[coveralls]

Coverage remained the same at 0.0% when pulling 166356b on Eric-Arellano:fix-related-subsystem-self-reference into 98f9ca1 on pantsbuild:master.