formula: trap CMake FetchContent usage instead of using FETCHCONTENT_FULLY_DISCONNECTED

[alebcay]

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Follow-up to #17075. With this change, when a formula attempts to use FetchContent_MakeAvailable, the following error is presented:

-- Configuring built-in curl...
CMake Error at /opt/homebrew/Library/Homebrew/trap_fetchcontent_provider.cmake:4 (message):
  Refusing to populate dependency 'zlib' with FetchContent while building in
  Homebrew, please add the dependency as a dependency or resource in the
  formula
Call Stack (most recent call first):
  /opt/homebrew/Cellar/cmake/3.29.3/share/cmake/Modules/FetchContent.cmake:2017:EVAL:1 (trap_fetchcontent_provider)
  /opt/homebrew/Cellar/cmake/3.29.3/share/cmake/Modules/FetchContent.cmake:2017 (cmake_language)
  cmake/zlib_external.cmake:15 (FetchContent_MakeAvailable)
  CMakeLists.txt:214 (include)


-- Configuring incomplete, errors occurred!

(This is using a modified version of the cpr formula to try the scenario where FetchContent_MakeAvailable gets used; normally this formula will rely on brewed dependencies.)

Formulae can opt out by adding flag -DCMAKE_PROJECT_TOP_LEVEL_INCLUDES= (essentially unsetting the variable). Maybe we can provide something less unwieldy/better named to accomplish this.

Seeking feedback especially about:

• The error message and what end users or formula authors can do about it
• Name and location of the .cmake file - for now I just dropped it in HOMEBREW_LIBRARY_PATH, maybe it makes sense to be in some subdirectory, or in the Cellar somewhere (include this file in the cmake formula/bottle rather than as part of brew)

[MikeMcQuaid]

Formulae can opt out by adding flag -DCMAKE_PROJECT_TOP_LEVEL_INCLUDES= (essentially unsetting the variable). Maybe we can provide something less unwieldy/better named to accomplish this.

Yeh, ideally there'd be a dedicated CMake variable for this -DHOMEBREW_ALLOW_FETCHCONTENT or something?

• The error message and what end users or formula authors can do about it

I think it's great!

[alebcay]

• Moved .cmake file into cmake subdirectory
• Added HOMEBREW_ALLOW_FETCHCONTENT option, which can be set to ON to bypass this
• Adjusted message text

[MikeMcQuaid]

Yeh, this looks perfect, thanks again @alebcay!

[alebcay]

Adjusted the scope of the conditional - we probably should just avoid creating the dependency provider altogether if we want to bypass.

[MikeMcQuaid]

Looks good, thanks again @alebcay!