PostgreSQL@11: Add datadir, PG_VERSION check, and log file helpers

Also remove migration notes from caveats.

Formula/postgresql@11.rb

@@ -71,19 +71,26 @@ class PostgresqlAT11 < Formula
     return if ENV["CI"]
 
     (var/"log").mkpath
-    (var/name).mkpath
-    unless File.exist? "#{var}/#{name}/PG_VERSION"
-      system "#{bin}/initdb", "--locale=C", "-E", "UTF-8", "#{var}/#{name}"
-    end
+    postgresql_datadir.mkpath
+    system "#{bin}/initdb", "--locale=C", "-E", "UTF-8", postgresql_datadir unless pg_version_exists?
+  end
+
+  def postgresql_datadir
+    var/name
+  end
+
+  def postgresql_log_path
+    var/"log/#{name}.log"
+  end
+
+  def pg_version_exists?
+    (postgresql_datadir/"PG_VERSION").exist?
   end
 
   def caveats
     <<~EOS
-      To migrate existing data from a previous major version of PostgreSQL run:
-        brew postgresql-upgrade-database
-
       This formula has created a default database cluster with:
-        initdb --locale=C -E UTF-8 #{var}/postgres
+        initdb --locale=C -E UTF-8 #{postgresql_datadir}
       For more details, read:
         https://www.postgresql.org/docs/#{version.major}/app-initdb.html
     EOS
@@ -105,16 +112,16 @@ class PostgresqlAT11 < Formula
         <array>
           <string>#{opt_bin}/postgres</string>
           <string>-D</string>
-          <string>#{var}/#{name}</string>
+          <string>#{postgresql_datadir}</string>
         </array>
         <key>RunAtLoad</key>
         <true/>
         <key>WorkingDirectory</key>
         <string>#{HOMEBREW_PREFIX}</string>
         <key>StandardOutPath</key>
-        <string>#{var}/log/#{name}.log</string>
+        <string>#{postgresql_log_path}</string>
         <key>StandardErrorPath</key>
-        <string>#{var}/log/#{name}.log</string>
+        <string>#{postgresql_log_path}</string>
       </dict>
       </plist>
     EOS