Fix error when installing over existing plugins

As reported in PICARD-444, the following error is received when
installing over an existing plugin:

Traceback (most recent call last):
File ".\picard\ui\options\plugins.py", line 134, in open_plugins
self.install_plugin(path)
File ".\picard\ui\options\plugins.py", line 148, in install_plugin
self.tagger.pluginmanager.install_plugin(path, dest)
File ".\picard\plugin.py", line 198, in install_plugin
same_file = os.path.samefile(path, dest) if dest_exists else False
AttributeError: 'module' object has no attribute 'samefile'

picard/plugin.py

@@ -25,6 +25,7 @@ import picard.plugins
 import traceback
 from picard import config, log
 from picard.const import USER_PLUGIN_DIR
+from picard.util import os_path_samefile
 
 
 _suffixes = [s[0] for s in imp.get_suffixes()]
@@ -195,7 +196,7 @@ class PluginManager(QtCore.QObject):
         if plugin_name:
             try:
                 dest_exists = os.path.exists(dest)
-                same_file = os.path.samefile(path, dest) if dest_exists else False
+                same_file = os_path_samefile(path, dest) if dest_exists else False
                 if os.path.isfile(path) and not (dest_exists and same_file):
                     shutil.copy(path, dest)
                 elif os.path.isdir(path) and not same_file:

picard/util/__init__.py

@@ -400,3 +400,11 @@ def tracknum_from_filename(base_filename):
     if numbers:
         return numbers[0]
     return -1
+
+if sys.platform == 'win32':
+    def os_path_samefile(p1, p2):
+        ap1 = os.path.abspath(p1)
+        ap2 = os.path.abspath(p2)
+        return ap1 == ap2
+else:
+    os_path_samefile = os.path.samefile