[assimp] Fix generated -config files to use more generic form

[ras0219]

Assimp has two sets of config files: one that properly handles dependencies and one that doesn't. This PR switches which set to install.

Additionally, this PR causes the embedded IrrXML source to be built directly into the assimp library instead of as a separate binary for simpler consumption.

• What does your PR fix? Fixes #
  Fixes [assimp] static link failure #13253 assimp build failure #12154 assimp package does not link irrXML #9918 assimp build failure using CMake #9022

[BillyONeal]

How does this compare to #12437 ?

[BillyONeal]

OK, this is fine to me but please sync with @JackBoosY on the other review.

[NancyLi1013]

The failures on x64-linux due to this:

CMake Error at scripts/cmake/vcpkg_fixup_cmake_targets.cmake:72 (message):
  '/mnt/vcpkg-ci/packages/assimp_x64-linux/debug/lib/cmake/assimp' does not
  exist.
Call Stack (most recent call first):
  ports/assimp/portfile.cmake:37 (vcpkg_fixup_cmake_targets)
  scripts/ports.cmake:79 (include)

[petersteneteg]

Assimps own hardcoded config files uses the target name "assimp::assimp" but with this change the name will be "Assimp::assimp" which breaks compatibility...

[ras0219]

I've added in a backwards-compatible alias that forwards assimp::assimp to Assimp::assimp. It's unfortunate that their cmake files have two different target names, but that's life I suppose.

This PR should now handle all cases covered in #12437. @JackBoosY Are there any remaining uncovered issues as far as you can tell?

[cenit]

In #12199 I had to modify assimp a lot, involving some of the things here

[JackBoosY]

Will test usage on Linux.

[JackBoosY]

Passed.

[BillyONeal]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[cenit]

May I ask you a comment about the fixes in #12199? IMHO those are very different and maybe more complete. These fixes here do not solve almost anything related to the problems I had there in using assimp

[cenit]

Also another comment about the note in the portfile: irrlicht contains irrxml, but technically that port should use “external” irrxml (which I added and worked in the above PR)

[Neumann-A]

hmmm this also broke #13464. So why is irrxml embedded and not an extra port?