Added blacklisting of implicit modules in docs + regression tests

fixes #4400

The feature requested in #4400 was already part of the module file
configuration, but it was neither tested nor documented. This
commit takes care of adding a few lines in the documentation and a
regression test.

lib/spack/docs/tutorial_modules.rst

@@ -555,6 +555,26 @@ you'll see that now the module for ``gcc@7.2.0`` has reappeared:
   Use "module keyword key1 key2 ..." to search for all possible modules matching any of the "keys".
 
 
+An additional possibility that you can leverage to unclutter the environment
+is that of preventing the generation of module files for implicitly installed
+packages. In this case all one needs to do is to add the following line:
+
+.. code-block:: yaml
+  :emphasize-lines: 3
+
+  modules:
+    tcl:
+      blacklist_implicits: true
+      whitelist:
+        -  gcc
+      blacklist:
+        -  '%gcc@5.4.0'
+      all:
+        filter:
+          environment_blacklist: ['CPATH', 'LIBRARY_PATH']
+
+to ``modules.yaml`` and regenerate the module file tree as above.
+
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 Change module file naming
 ^^^^^^^^^^^^^^^^^^^^^^^^^

lib/spack/spack/schema/modules.py

@@ -121,6 +121,10 @@
                         '$ref': '#/definitions/array_of_strings'},
                     'blacklist': {
                         '$ref': '#/definitions/array_of_strings'},
+                    'blacklist_implicits': {
+                        'type': 'boolean',
+                        'default': False
+                    },
                     'naming_scheme': {
                         'type': 'string'  # Can we be more specific here?
                     }

lib/spack/spack/test/conftest.py

@@ -327,7 +327,7 @@ def _populate(mock_db):
     def _install(spec):
         s = spack.spec.Spec(spec).concretized()
         pkg = spack.repo.get(s)
-        pkg.do_install(fake=True)
+        pkg.do_install(fake=True, explicit=True)
 
     # Transaction used to avoid repeated writes.
     with mock_db.write_transaction():

lib/spack/spack/test/data/modules/tcl/blacklist_implicits.yaml

@@ -0,0 +1,6 @@
+enable:
+  - tcl
+tcl:
+  blacklist_implicits: true
+  all:
+    autoload: 'direct'

lib/spack/spack/test/modules/tcl.py

@@ -274,3 +274,24 @@ def test_extend_context(
 
         short_description = 'module-whatis "This package updates the context for TCL modulefiles."'  # NOQA: ignore=E501
         assert short_description in content
+
+    @pytest.mark.regression('4400')
+    @pytest.mark.db
+    def test_blacklist_implicits(
+            self, modulefile_content, module_configuration, database
+    ):
+        module_configuration('blacklist_implicits')
+
+        # mpileaks has been installed explicitly when setting up
+        # the tests database
+        mpileaks_specs = database.query('mpileaks')
+        for item in mpileaks_specs:
+            writer = writer_cls(item)
+            assert not writer.conf.blacklisted
+
+        # callpath is a dependency of mpileaks, and has been pulled
+        # in implicitly
+        callpath_specs = database.query('callpath')
+        for item in callpath_specs:
+            writer = writer_cls(item)
+            assert writer.conf.blacklisted