MixinLibraries - Restore support for writing *.phar files

[totten]

Overview

Update the way in which {$ext}/mixin/lib/ is populated. Specifically, given that we want to backport something from civix:lib/, how will we write the file?

(Over time, this topic is a bit flip-floppy. The impetus behind the present change is that CiviCRM 6.1+ is more broadly deployed now. So "what works" is shifting.)

(cc @colemanw; related to discussion on #427.)

Before + After

• Original Behavior: Create a backport-file {$ext}/mixin/lib/civimix-schema@X.Y.Z.phar
  • When support for mixin-libraries was first introduced ((dev/core#4999) Support Entity Framework v2 for extensions #331), the backport tool would simply copy() the library.
  • This turned out to be problematic when deploying on Backdrop, because Backdrop completely disabled PHAR support in the runtime. A long-term fix was merged into civicrm-core 6.1 (Enable PHAR loading on Backdrop CMS (web UI) civicrm/civicrm-core#32310), and a mid-term fix was applied to civix (Convert civimix-schema@5 from PHAR to subdir #387), which leads to the current status quo...
• Status Quo Behavior: Create backport-folder {$ext}/mixin/lib/civimix-schema@X.Y.Z/
• New Behavior: Create the file or the folder, depending on your target <compatibility>.
  • If your extension targets CiviCRM v6.1 or newer, then create file {$ext}/mixin/lib/civimix-schema@X.Y.Z.phar
  • If your extension targets CiviCRM v6.0 or older, then create folder {$ext}/mixin/lib/civimix-schema@X.Y.Z/