"import spot.foo" may now load "spot-extra/foo.py"

This is needed for tcltl.

* python/spot/__init__.py: Alter __path__ to add any spot-extra/
directory we find.
* NEWS: Mention this.

NEWS

@@ -2,6 +2,12 @@ New in spot 2.8.2.dev  (not yet released)
 
   Nothing yet.
 
+  Python:
+
+  - Doing "import spot.foo" will now load any spot-extra/foo.py on
+    Python's search path.  This can be used to install third-party
+    packages that want to behave as plugins for Spot.
+
 New in spot 2.8.2 (2019-09-27)
 
   Command-line tools:

python/spot/__init__.py

@@ -36,6 +36,16 @@ if 'SPOT_UNINSTALLED' in os.environ:
     # into .libs/
     __path__.extend(sys.path)
 
+# We may have third-party plugins that want to be loaded as "spot.xxx", but
+# that are installed in a different $prefix.  This sets things so that any
+# file that looks like spot-extra/xxx.py can be loaded with "import spot.xxx".
+for path in sys.path:
+    if path not in __path__:
+        path += "/spot-extra"
+        if os.path.isdir(path):
+            __path__.append(path)
+
+
 from spot.impl import *
 from spot.aux import \
      extend as _extend, \