Update a supported package 'user-meamc' for lammps (#17207)

* Update package.py

* edit confliction when add package 'meam' 

 The USER-MEAMC fully replaces the MEAM package, which has been removed from LAMMPS after the 12 December 2018 version.

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

@@ -61,8 +61,8 @@ def url_for_version(self, version):
                           'molecule', 'mpiio', 'peri', 'poems', 'python',
                           'qeq', 'replica', 'rigid', 'shock', 'snap', 'spin',
                           'srd', 'user-atc', 'user-h5md', 'user-lb',
-                          'user-misc', 'user-netcdf', 'user-omp', 'user-reaxc',
-                          'voronoi']
+                          'user-meamc', 'user-misc', 'user-netcdf', 'user-omp',
+                          'user-reaxc', 'voronoi']
 
     for pkg in supported_packages:
         variant(pkg, default=False,
@@ -120,6 +120,8 @@ def url_for_version(self, version):
     conflicts('+user-misc', when='~manybody')
     conflicts('%gcc@9:', when='@:20200303+openmp')
     conflicts('+kokkos', when='@:20200227')
+    conflicts('+meam', when='@20181212:')
+    conflicts('+user-meamc', when='@:20181212')
 
     patch("lib.patch", when="@20170901")
     patch("660.patch", when="@20170922")