proper packages for mercurial_setup;

--- a/src/Pure/Tools/phabricator.scala	Fri Oct 30 23:43:08 2020 +0100
+++ b/src/Pure/Tools/phabricator.scala	Sat Oct 31 11:50:09 2020 +0100
@@ -28,13 +28,17 @@
       "git", "mysql-server", "apache2", "libapache2-mod-php", "php", "php-mysql",
       "php-gd", "php-curl", "php-apcu", "php-cli", "php-json", "php-mbstring",
       // more packages
-      "php-xml", "php-zip", "python-pygments", "ssh", "subversion",
+      "php-xml", "php-zip", "python-pygments", "ssh", "subversion", "python-pygments",
       // mercurial build packages
-      "make", "gcc", "python", "python-dev", "python-docutils", "python-pygments", "python-openssl")
+      "make", "gcc", "python", "python-dev", "python-docutils", "python-openssl")
 
   val packages_ubuntu_20_04: List[String] =
-    packages_ubuntu_18_04.map((name: String) =>
-      if (name.startsWith("python")) name.replace("python", "python3") else name)
+    packages_ubuntu_18_04.map(
+      {
+        case "python-pygments" => "python3-pygments"
+        case "python-dev" => "python2-dev"
+        case name => name
+      })
 
   def packages: List[String] =
   {