zlib-api: new virtual with zlib/zlib-ng as providers (#37372)

Introduces a new virtual zlib-api, which replaces zlib in most packages.

This allows users to switch to zlib-ng by default for better performance.

lib/spack/docs/packaging_guide.rst

@@ -2243,7 +2243,7 @@ looks like this:
        url      = "http://www.openssl.org/source/openssl-1.0.1h.tar.gz"
 
        version("1.0.1h", md5="8d6d684a9430d5cc98a62a5d8fbda8cf")
-       depends_on("zlib")
+       depends_on("zlib-api")
 
        parallel = False
 

lib/spack/spack/test/data/unparse/llvm.txt

@@ -235,7 +235,7 @@ class Llvm(CMakePackage, CudaPackage):
     depends_on("libffi", when="+cuda")  # libomptarget
 
     # llvm-config --system-libs libraries.
-    depends_on("zlib")
+    depends_on("zlib-api")
 
     # lldb dependencies
     depends_on("swig", when="+lldb")