ninja: drop python as a run dep (#11347)

* ninja: Make python a build-only dependency

* ninja, ninja-fortran: allow the packages to provide eachother

var/spack/repos/builtin/packages/ninja-fortran/package.py

@@ -25,7 +25,9 @@ class NinjaFortran(Package):
     version('1.7.2.0.gcc0ea', '3982f508c415c0abaca34cb5e92e711a')
     version('1.7.1.0.g7ca7f', '187a8d15c1e20e5e9b00c5c3f227ca8a')
 
-    depends_on('python', type=('build', 'run'))
+    depends_on('python', type='build')
+
+    provides('ninja')
 
     phases = ['configure', 'install']
 

var/spack/repos/builtin/packages/ninja/package.py

@@ -22,7 +22,9 @@ class Ninja(Package):
     version('1.7.2', '7b482218757acbaeac4d4d54a3cd94e1')
     version('1.6.0', '254133059f2da79d8727f654d7198f43')
 
-    depends_on('python', type=('build', 'run'))
+    depends_on('python', type='build')
+
+    provides('ninja-fortran', when='@kitware')
 
     phases = ['configure', 'install']