Per PMS 11.1 [1, note: wording improved in git master] the directory pointed by FILESDIR must not be accessed from global scope of an ebuild and in pkg_* functions. The value of this variable is only provided for setting variables such as PATCHES that are used later in src_* phases. [1]:https://projects.gentoo.org/pms/latest/pms.html#x1-11800011.1