meson: Fix Vulkan dependency checking

meson: Fix Vulkan dependency checking

The dependency block was completely wrong. It was:

1. Not searching for the lib manually when -Dvulkan=enabled (default).
   The else block was only hit when -Dvulkan=auto.
2. Unconditionally searching for the vulkan library in the else block
   when -Dvulkan=disabled

The manual searching is also not required because Meson has a custom
'vulkan' dependency class that already supports Windows, and is more
correct than the code here. Specifically, the current code does not
support picking up the Vulkan SDK from a custom path.

Fixes #3108