Simplify remoteExtPath regex

The complexity of this regular expression is just not needed. Just
search for the first occurrence of `/vendor/` or `/extensions/`
and match everything from there, that's it.

Using a regex character (`+`) as delimiter does have the advantage
that it must not be repeated when calling `preg_quote`, it's always
quoted.

Change-Id: Icbcc490879779bd6de64856411fcd862454347d4