SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.
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.
