Allow ports to customize the launchd plist files in the destroot phase

[ryandesign (Ryan Carsten Schmidt)]

Currently MacPorts creates any launchd plists for startupitem / startupitems keywords after the destroot phase is completely over, even after any post-destroot blocks have run. This provides no opportunity for portfiles to customize the launchd plist. Many ports have a desire to do so and have had to resort to inelegant hacks like modifying the installed plist in a post-activate block.

Can we instead create the plist before the destroot phase runs, or after the main destroot command runs but before the post-destroot blocks?

[ryandesign (Ryan Carsten Schmidt)]

In 6f7ed40b1df74c6dd62588a0f8cec5a7e8a6959e/macports-base (master):

Make startupitems before destroot phase, not after

This allows ports to customize their launchd plists during the destroot
phase. It also makes it easier for a developer to debug launchd plists
since now the entire destroot phase doesn't need to be completed before
the launchd plists are created.

Closes: #60653