GN: Componentize vulkan back-end build. This moves the build configuration into the Vulkan back-end dir. This should be a little easier to maintain as all Vulkan-related config is in one place. Note that this should not interfere with Skia's build as they do not import the Vulkan back-end sources. One additional possiblity that this enables is testing other compile-time permutations of the Vulkan back-end more easily. For example we could make a simple change to enable compile testing of the Vulkan back-end with custom command buffers disabled. Also fixes a few errors affecting less tested configs. Bug: angleproject:3943 Change-Id: Iaf819936896e4f5d3e6415ed16ab0c940e46cdb6 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1829662 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Tobin Ehlis <tobine@google.com>