Spack find: fix queries that specify dependencies (#14757)

Fixes #10019

If multiple instances of a package were installed in a single
instance of Spack, and they differed in terms of dependencies, then
"spack find" would not distinguish specs based on their dependencies.
For example if two instances of X were installed, one with Y and one
with Z, then "spack find X ^Y" would display both instances of X.

var/spack/repos/builtin.mock/packages/hdf5/package.py

@@ -0,0 +1,15 @@
+# Copyright 2013-2020 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+
+class Hdf5(Package):
+    homepage = "http://www.llnl.gov"
+    url      = "http://www.llnl.gov/hdf5-1.0.tar.gz"
+
+    version(2.3, 'foobarbaz')
+
+    variant('mpi', default=True, description='Debug variant')
+
+    depends_on('mpi', when='mpi')