Environments: add run deps to shell modifications (#23485)

When adding an Environment to a user's shell, Spack was only adding
root specs. This now includes run dependencies of root specs.

var/spack/repos/builtin.mock/packages/depends-on-run-env/package.py

@@ -0,0 +1,22 @@
+# Copyright 2013-2021 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)
+
+from spack import *
+
+
+class DependsOnRunEnv(Package):
+    """This package has a runtime dependency on another package which needs
+       to perform shell modifications to run.
+    """
+
+    homepage = "http://www.example.com"
+    url      = "http://www.example.com/a-1.0.tar.gz"
+
+    version('1.0', '0123456789abcdef0123456789abcdef')
+
+    depends_on('modifies-run-env', type=('run',))
+
+    def install(self, spec, prefix):
+        mkdirp(prefix.bin)

var/spack/repos/builtin.mock/packages/modifies-run-env/package.py

@@ -0,0 +1,21 @@
+# Copyright 2013-2021 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)
+
+from spack import *
+
+
+class ModifiesRunEnv(Package):
+    """Dependency package which needs to make shell modifications to run"""
+
+    homepage = "http://www.example.com"
+    url      = "http://www.example.com/a-1.0.tar.gz"
+
+    version('1.0', '0123456789abcdef0123456789abcdef')
+
+    def setup_run_environment(self, env):
+        env.set('DEPENDENCY_ENV_VAR', '1')
+
+    def install(self, spec, prefix):
+        mkdirp(prefix.bin)