OpenGL restructure (#31748)

* OpenGL: Restructures the OpenGL packages

This provides concrete glx and osmesa packages which delegate to
virtual libglx and libosmesa packages provided by mesa. This was
necessary because GLX and OSMesa both provide gl implementations but
with mesa providing the girtual gl package there was no way to properly
distinguish which of the two opengl implementations was beiing requested
when querying the spec['gl'] dependency.  This additional level of
indirection allows for that.

* OpenGL: Adjust downstream dependents of OpenGL for the restructure

This implements the necessary fixes in the packages that depend on
OpenGL to work with the restructuring.  This also attempts to create a
consistent variant for specifying glx or osmesa.

etc/spack/defaults/packages.yaml

@@ -25,16 +25,17 @@ packages:
       fftw-api: [fftw, amdfftw]
       flame: [libflame, amdlibflame]
       fuse: [libfuse]
-      gl: [mesa+opengl, mesa18, opengl]
+      gl: [glx, osmesa]
       glu: [mesa-glu, openglu]
-      glx: [mesa+glx, mesa18+glx, opengl]
       golang: [gcc]
       iconv: [libiconv]
       ipp: [intel-ipp]
       java: [openjdk, jdk, ibm-java]
       jpeg: [libjpeg-turbo, libjpeg]
       lapack: [openblas, amdlibflame]
-      libllvm: [llvm, llvm-amdgpu]
+      libglx: [mesa+glx, mesa18+glx]
+      libllvm: [llvm]
+      libosmesa: [mesa+osmesa, mesa18+osmesa]
       lua-lang: [lua, lua-luajit-openresty, lua-luajit]
       luajit: [lua-luajit-openresty, lua-luajit]
       mariadb-client: [mariadb-c-client, mariadb]
@@ -44,7 +45,6 @@ packages:
       mysql-client: [mysql, mariadb-c-client]
       opencl: [pocl]
       onedal: [intel-oneapi-dal]
-      osmesa: [mesa+osmesa, mesa18+osmesa]
       pbs: [openpbs, torque]
       pil: [py-pillow]
       pkgconfig: [pkgconf, pkg-config]