en-GB language pack package correction (fix inconsistency)

[andrepereiradasilva]

Pull Request for New Issue.

Summary of Changes

As detected in PR joomla#9744, there is an inconsistency in the en-GB language extensions.

The default language (en-GB) should have 3 extensions like any other language pack:

1. type = language | client = Site: For the site site language
2. type = language | client = Administrator: For the administrator site language
3. type = package | client = Site: For the whole en-GB language package

But, only extensions "1." (en-GB site language) and "2." (en-GB admin language) exist in Joomla core default database.

The extension "3." (pkg_en-GB language package), although is has a manifest file, doesn't have an extension in Joomla core default database.

Note that this is the only manifest file in the whole Joomla core that doesn't has an extension in the database, and for consistency it should.

This generates some inconsistencies, for instance the update server is declared in the extension "3." (en-GB language package) manifest file, but in the database is linked to extension "1." (en-GB site language).
Extension "1." (en-GB site language) doesn't have update servers in his manifest file, because is a extension of language type.

So, this PR adds the "pkg_en-GB language package" extension to new joomla installation, also adds it to the upgrade process and makes the necessary adjustments in the code.

Testing Instructions

New installation

1. Install Joomla with this package https://github.com/andrepereiradasilva/joomla-cms/archive/en-gb-package-correc.zip
2. Add some languages in the end of the installation. Check if they are installed correctly.
3. Go to admin. "Extensions -> Manage -> Install languages"
4. Install some languages. Check if everything is fine
5. Go to "Extensions -> Manage -> Update Sites" and check if all "Accredited Joomla! Translations" are o the type "Package", including the en-GB one.
6. Go to "Extensions -> Manage -> Manage", filter by package and check if and languages are there. Including the en-GB (should be blocked).

Upgrade

1. Use 3.5.1 or latest staging
2. Install patch Joomla! Update: reinstall Joomla! and Update & Install features | seccond try joomla/joomla-cms#9612 so you can upload joomla with upload
3. Go to Joomla update component
4. Update joomla with this custom 3.5.5 package https://github.com/andrepereiradasilva/update.joomla.org/raw/master/Joomla_3.5.1_to_3.5.5-Stable-Patch_Package.zip
5. Repeat the same steps "3." to "6." of the "New installation" test.