builder: Allow .pyc files without .py

This is common for binary-only releases, for example as described
in:
https://lists.freedesktop.org/archives/flatpak/2017-June/000697.html

tests/org.test.Python2.json

@@ -0,0 +1,37 @@
+{
+    "app-id": "org.test.Python2",
+    "runtime": "org.test.PythonPlatform",
+    "sdk": "org.test.PythonSdk",
+    "command": "testpython.py",
+    "modules": [
+        {
+            "name": "compiled-python",
+            "post-install": [
+                "mkdir /app/bin",
+                "cp testpython.py /app/bin",
+                "cp importme.py /app/bin",
+                /* Compile importme.py */
+                "/app/bin/testpython.py",
+                /* Remove .py file, but keep .pyc */
+                "rm  /app/bin/importme.py",
+                /* Make sure it still works */
+                "/app/bin/testpython.py"
+            ],
+            "sources": [
+                {
+                    "type": "file",
+                    "path": "empty-configure",
+                    "dest-filename": "configure"
+                },
+                {
+                    "type": "file",
+                    "path": "testpython.py"
+                },
+                {
+                    "type": "file",
+                    "path": "importme.py"
+                }
+            ]
+        }
+    ]
+}