swc-installation-test-2.py: Add support for package-specific URLs

To give users more targeted advice for fixing their installation
problems, we should link them to somewhere more specific than the root
SWC setup page.  The two knobs in the specific URL are system/OS
(Gentoo, Debian, OS X, MS Windows, ...) and package (the dependency
name: emacs, git, virtual-browser, ...).  We don't have consistent
URLs upstream yet [1], but maybe this commit will help motivate them
;).

[1]: https://github.com/swcarpentry/website/issues/2

setup/swc-installation-test-2.py

@@ -45,6 +45,10 @@ def import_module(name):
 import shlex as _shlex
 import subprocess as _subprocess
 import sys as _sys
+try:  # Python 3.x
+    import urllib.parse as _urllib_parse
+except ImportError:  # Python 2.x
+    import urllib as _urllib_parse  # for quote()
 
 
 if not hasattr(_shlex, 'quote'):  # Python versions older than 3.3
@@ -107,6 +111,12 @@ def __str__(self):
 
 
 class DependencyError (Exception):
+    _system_map = {  # map long system names to shorter forms
+        'Gentoo Base System': 'Gentoo',
+        }
+    _supported = [  # (system, package) pairs with specific instructions
+        ]
+
     def _get_message(self):
         return self._message
     def _set_message(self, message):
@@ -121,8 +131,22 @@ def __init__(self, checker, message, causes=None):
             causes = []
         self.causes = causes
 
+    def get_url(self):
+        url = 'http://software-carpentry.org/setup/'
+        system = _platform.system()
+        if system == 'Linux':
+            system = _platform.linux_distribution()[0] or system
+        system = self._system_map.get(system, system)
+        package = self.checker.name
+        if (system, package) in self._supported:
+            url = '{0}{1}.html#{2}'.format(
+                url,
+                _urllib_parse.quote(system.lower()),
+                _urllib_parse.quote(package))
+        return url
+
     def __str__(self):
-        url = 'http://software-carpentry.org/setup/'  # TODO: per-package URL
+        url = self.get_url()
         lines = [
             'check for {0} failed:'.format(self.checker.full_name()),
             '  ' + self.message,