Checkmk Werk 13224: DEV: make legacy includes packageable

ID: 13224 Title: DEV: make legacy includes packageable Component: Checks & agents Level: 1 Class: New feature Version: 2.1.0i1 This only affects plugin developers. With this werk, changes to files in <tt>cmk/base/check_legacy_includes</tt> can now be included in MKPs. The files in the sites <tt>local</tt> folder will be considered, and win over their shipped counterparts. This has been made necessary to backport various comissioned features or bug fixes. <b>Note:</b> This is, as the name suggests, all legacy code. The <tt>check_legacy_includes</tt> and its contents are <b>deprecated</b> and we strongly advise against using it.