[Backport release-2.18] Always use FetchContent to get vcpkg.

[teo-tsirpanis]

Backport 1f4ed0a from #4484.

---

TYPE: BUILD
DESC: Remove the externals/vcpkg submodule. Vcpkg is always downloaded by CMake.

[teo-tsirpanis]

Fixes vcpkg initialization failures under certain circumstances¹ due to a typo in #4529². Verified on my WSL environment.

This is a more complete backport of #4484. Instead of choosing between either the submodule or FetchContent we always use FetchContent, making things more consistent and predictable. #4484 has been in dev for almost a month with no related failures.

If CI succeeds it should be OK; the same code is now used regardless of whether we get TileDB from a Git repository or a tarball.

Footnotes

1. If we build from a non-Git repository and there is a .git directory two levels above the source directory. ↩

2. This line should have said CMAKE_CURRENT_LIST_DIR. ↩

[ihnorton]

I think the underlying issue fixed here caused the failure in conda-forge/tiledb-feedstock#218

(@teo-tsirpanis please confirm, so we can skip that conda build if so)