Use a better technique for overriding the .ddev/xhprof/xhprof_prepend.php

[rfay]

Expected Behavior

One of our goals this year is to have improving and easier-to-use support for xhprof via xhgui, including full support for @tyler36 's https://github.com/tyler36/ddev-xhgui

• Use XHGui as front-end for xhprof #4606
• XHProf - Wishlist: find logs, persist runs #3782
• Unwilling to remove 'xhprof_prepend.php' because it does not have #ddev-generated in it:  tyler36/ddev-xhgui#11

In order to do that, one of the key factors is the awkwardness of overriding xhprof_prepend.php. That has "just been that way" from the beginning, and we can use another technique for it.

Actual Behavior

It's hard to override properly

Steps To Reproduce

No response

Anything else?

No response