genpolicy tweaks (#141)

Allow `import input` instead of erroring out.
This import is redundant and has no effect.

Emit `print` messages to stderr onstead of stdout.

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

tests/interpreter/cases/import/tests.yaml

@@ -71,3 +71,11 @@ cases:
         import foo
     query: data
     error: "import path must begin with one of"
+
+  - note: redundant import input
+    modules:
+      - |
+        package test
+        import input
+    query: data.test
+    want_result: {}