allow concat::fragment target param to be an arbitrary string

Revert validation of the target param as an absolute path and allow it to be an
arbitrary string.  This is so the

    concat { <foo>: path => ...  }
    concat::fragment { ...: target => <foo> }

association may be symbolic as long as concat path param is specified.  This
should resolve the symbolic name regression introduced in:
https://github.com/puppetlabs/puppetlabs-concat/commit/eaf84079