May a natural person who enters into a partnership contract with other natural persons, a partnership without legal personality that is not declared or registered for tax purposes, intended for the execution of future works (building) on a site forming part of the personal assets of some of the contracting parties be regarded, in the light of the circumstances of the main case, as a taxable person for VAT purposes within the meaning of Article 9(1) of the VAT Directive, where, under the fiscal rules, the tax authorities initially treated the transfer of ownership of the buildings on the site forming part of the personal assets of some of the contracting parties as sales falling within the scope of the management of the private wealth of such persons?