The pack is already has its validity checked at the beginning of the
function, so we don't need to check this again after inserting it.
Also resolves a use-after-move case.
This allows the same code to be used to read into a std::string, which
allows for eliminating the vector->string transfer when reading the
manifest file.
A ContiguousContainer is a concept that includes std::array,
std::string, and std::vector.
Makes it way harder to introduce resource leaks, and plugs the existing
resource leaks in the constructor and Install() where the file wouldn't
be closed in some error cases.
