Use composer installed phpseclib

We used to ship only parts of phpseclib in inc/phpseclib which made it a
little tricky to update. This replaces the library with a composer
installed version, making

Use composer installed phpseclib

We used to ship only parts of phpseclib in inc/phpseclib which made it a
little tricky to update. This replaces the library with a composer
installed version, making the rest of the library available to plugin
authors. It might fix a problem with PHP 7.1 as reported in #1682
(unconfirmed)

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