bugfix: allow fetching no-code packages (#13429)

Previously, spack would error out if we tried to fetch something with no
code, but that would prevent fetching dependencies. In particular, this
would fail:

    spack fetch --dependencies xsdk

- [x] Instead of raising an error, just print a message that there is nothing
      to be fetched for packages like xsdk that do not have code.

- [x] Make BundleFetchStrategy a bit more quiet about doing nothing.

lib/spack/spack/fetch_strategy.py

@@ -201,28 +201,16 @@ class BundleFetchStrategy(FetchStrategy):
     url_attr = ''
 
     def fetch(self):
-        tty.msg("No code to fetch.")
+        """Simply report success -- there is no code to fetch."""
         return True
 
-    def check(self):
-        tty.msg("No code to check.")
-
-    def expand(self):
-        tty.msg("No archive to expand.")
-
-    def reset(self):
-        tty.msg("No code to reset.")
-
-    def archive(self, destination):
-        tty.msg("No code to archive.")
-
     @property
     def cachable(self):
-        tty.msg("No code to cache.")
+        """Report False as there is no code to cache."""
         return False
 
     def source_id(self):
-        tty.msg("No code to be uniquely identified.")
+        """BundlePackages don't have a source id."""
         return ''
 
 

lib/spack/spack/package.py

@@ -1046,7 +1046,9 @@ def do_fetch(self, mirror_only=False):
             raise ValueError("Can only fetch concrete packages.")
 
         if not self.has_code:
-            raise InvalidPackageOpError("Can only fetch a package with a URL.")
+            tty.msg(
+                "No fetch required for %s: package has no code." % self.name
+            )
 
         start_time = time.time()
         checksum = spack.config.get('config:checksum')

lib/spack/spack/test/install.py

@@ -9,8 +9,7 @@
 
 from llnl.util.filesystem import mkdirp, touch, working_dir
 
-from spack.package import \
+from spack.package import InstallError, PackageBase, PackageStillNeededError
-    InstallError, InvalidPackageOpError, PackageBase, PackageStillNeededError
 import spack.patch
 import spack.repo
 import spack.store
@@ -327,7 +326,9 @@ def test_uninstall_by_spec_errors(mutable_database):
         PackageBase.uninstall_by_spec(rec.spec)
 
 
-def test_nosource_pkg_install(install_mockery, mock_fetch, mock_packages):
+@pytest.mark.disable_clean_stage_check
+def test_nosource_pkg_install(
+        install_mockery, mock_fetch, mock_packages, capfd):
     """Test install phases with the nosource package."""
     spec = Spec('nosource').concretized()
     pkg = spec.package
@@ -336,9 +337,8 @@ def test_nosource_pkg_install(install_mockery, mock_fetch, mock_packages):
     pkg.do_install()
 
     # Also make sure an error is raised if `do_fetch` is called.
-    with pytest.raises(InvalidPackageOpError,
+    pkg.do_fetch()
-                       match="fetch a package with a URL"):
+    assert "No fetch required for nosource" in capfd.readouterr()[0]
-        pkg.do_fetch()
 
 
 def test_nosource_pkg_install_post_install(