meson: use allow_fallback instead of manually listing the deps and what they provide

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26834>

meson.build

@@ -1526,7 +1526,7 @@ else
 endif
 
 dep_zlib = dependency('zlib', version : '>= 1.2.3',
-                      fallback : ['zlib', 'zlib_dep'],
+                      allow_fallback: true,
                       required : get_option('zlib'))
 if dep_zlib.found()
   pre_args += '-DHAVE_ZLIB'
@@ -1569,7 +1569,7 @@ with_expat = get_option('expat') \
 if host_machine.system() == 'darwin'
   dep_expat = meson.get_compiler('c').find_library('expat', required : with_expat)
 else
-  dep_expat = dependency('expat', fallback : ['expat', 'expat_dep'],
+  dep_expat = dependency('expat', allow_fallback: true,
                          required : with_expat)
 endif