[CORE] Move core plugins to a new modules directory

For reference (raised by rozzin in IRC):

* http://foldoc.org/module
* http://foldoc.org/library
* http://foldoc.org/plugin

As noted by XRevan86, modules are not necessarily non-essential.
As we will keep the modules directory in GS root [therefore, near to
plugins/], it is evidenced the difference between both.

This is a simple yet fundamental structural change. It doesn't change
functionality but makes clearer the way we understand GNU social's
internals.

plugins/Embed/locale/Embed.pot

@@ -8,7 +8,7 @@ msgid ""
 msgstr ""
 "Project-Id-Version: PACKAGE VERSION\n"
 "Report-Msgid-Bugs-To: \n"
-"POT-Creation-Date: 2019-06-10 00:39+0100\n"
+"POT-Creation-Date: 2019-08-14 14:44+0100\n"
 "PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n"
 "Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
 "Language-Team: LANGUAGE <LL@li.org>\n"
@@ -17,13 +17,13 @@ msgstr ""
 "Content-Type: text/plain; charset=CHARSET\n"
 "Content-Transfer-Encoding: 8bit\n"
 
-#. TRANS: Plugin description.
-#: OembedPlugin.php:721
-msgid "Plugin for using and representing Oembed data."
-msgstr ""
-
 #. TRANS: Exception. %s is the URL we tried to GET.
-#: lib/oembedhelper.php:93
+#: lib/embedhelper.php:87
 #, php-format
 msgid "Could not GET URL %s."
 msgstr ""
+
+#. TRANS: Plugin description.
+#: EmbedPlugin.php:634
+msgid "Plugin for using and representing oEmbed, OpenGraph and other data."
+msgstr ""