`ExternalProject_Add` treats all keyword arguments as multi-valued.
FetchContent re-orders some keyword arguments w.r.t. unparsed arguments.
Add an internal keyword argument to separate the keyword values from
following unparsed arguments.

This was exposed by commit 550f6344 (ExternalProject/FetchContent:
Support relative remote URLs, 2023-04-21, v3.27.0-rc1~156^2) by adding
`GIT_REPOSITORY` to the set of re-ordered keyword arguments.  This
caused its value to absorb following unparsed arguments.

Fixes: #25029