Packages must be added to DB to be considered installed (#8038)

Fixes #8036

Before this PR Package.installed was returning True if the spec prefix
existed, without checking the DB. This is wrong for external packages,
whose prefix exists before being registered into the DB. Now the property
checks for both the prefix and a DB entry.

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

@@ -30,6 +30,7 @@ class Externaltool(Package):
     url      = "http://somewhere.com/tool-1.0.tar.gz"
 
     version('1.0', '1234567890abcdef1234567890abcdef')
+    version('0.9', '1234567890abcdef1234567890abcdef')
 
     depends_on('externalprereq')