root/intel-tbb interaction (#22366)

root 6.22 does not work with intel-tbb 2021.1.1.

So:
* Introduce conflicts()
  (a fitting depends_on in the comments, but does not help with the classic conretizer.)
* Mark 2020.3 as preferred to help concretizer, when newer versions come up.
* Previous discussion:
  https://github.com/spack/spack/pull/22263#issuecomment-797606475
* Relevant ROOT issue:
  https://github.com/root-project/root/issues/6933

var/spack/repos/builtin/packages/intel-tbb/package.py

@@ -23,7 +23,8 @@ class IntelTbb(Package):
     # Note: when adding new versions, please check and update the
     # patches, filters and url_for_version() below as needed.
 
-    version('2020.3', sha256='ebc4f6aa47972daed1f7bf71d100ae5bf6931c2e3144cf299c8cc7d041dca2f3')
+    version('2020.3', sha256='ebc4f6aa47972daed1f7bf71d100ae5bf6931c2e3144cf299c8cc7d041dca2f3',
+            preferred=True)
     version('2020.2', sha256='4804320e1e6cbe3a5421997b52199e3c1a3829b2ecb6489641da4b8e32faf500')
     version('2020.1', sha256='7c96a150ed22bc3c6628bc3fef9ed475c00887b26d37bca61518d76a56510971')
     version('2020.0', sha256='57714f2d2cf33935db33cee93af57eb3ecd5a7bef40c1fb7ca4a41d79684b118')

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

@@ -248,6 +248,10 @@ class Root(CMakePackage):
     depends_on('shadow',    when='+shadow')
     depends_on('sqlite',    when='+sqlite')
     depends_on('tbb',       when='+tbb')
+    # See: https://github.com/root-project/root/issues/6933
+    conflicts('^intel-tbb@2021.1:', when='@:6.22',
+              msg='Please use an older intel-tbb version')
+    # depends_on('intel-tbb@:2021.0', when='@:6.22 ^intel-tbb')
     depends_on('unuran',    when='+unuran')
     depends_on('vc',        when='+vc')
     depends_on('vdt',       when='+vdt')