Fix two "implode()" order issues arising from wilds/experimental collapse

Summary:
Ref T13490. There's one simple "implode()" order issue here, and one slightly more complex one that uses "DIRECTORY_SEPARATOR" as glue.

Add test coverage for this, update the lint check to detect constants used as glue, and fix the callsites.

Test Plan:

• Added a failing test and made it pass.
• Ran arc lint --everything and looked for remaining implode warnings, found none.

Maniphest Tasks: T13490

Differential Revision: https://secure.phabricator.com/D21024