Changes to pre-downloaded external dependencies

[Lestropie]

From Reviewing #2979, but listing separately as it's not critical there.

• For Eigen, ideally do a search across different zip / tarball extensions, so that it works regardless of which may have been downloaded; or alternatively require that the tarball have been pre-extracted in that directory (maybe more sensible given points below)

• For JSON for Modern C++, just expect the .hpp file to be in that directory; their GitHub offers a direct download to that one file

• Increment half dependency to 2.2.0, and as with JSON just check for the presence of that one file

• For the NIfTI headers, I wouldn't demand that they be placed in their own "nifti/ sub-directory; just expect both files to be present in the specified directory.
  Edit: As per response, may be better if, rather than the current state where only the NIfTI headers get their own sub-directory, all third-party dependencies have their own individualised directory within that pre-downloaded & pre-extracted dependency directory, even if some may contain one file only.

[daljit46]

I think the cleanest way to go about this would be to have separate directories for each dependency. This provides better encapsulation and allows for CMake targets to only include headers that they link against (e.g. if we place all headers in the same directory, then a target linking against half::half will have access to all the other headers too). Not really a big issue, but I believe it's less idiomatic than having separate directories per dependency.