Include project headers

[ken-matsui]

Discussed in https://github.com/orgs/poac-dev/discussions/759

^{Originally posted by oraqlle May 7, 2023}
It seems that headers cannot be found within a Poac package. How does one specify headers to Poac?

Current project structure

current-project
├── LICENSE
├── README.md
├── include
│   └── header.hpp
├── poac.toml
└── src
    └── main.cpp

Output

$ poac run -r      
   Compiling 1/1: current-project v0.1.0 ("/home/user/current-project")
FAILED: /home/user/current-project/poac-out/release/current-project 
g++ -std=c++20     src/main.cpp -o /home/user/current-project/poac-out/release/current-project
src/main.cpp:3:10: fatal error: header.hpp: No such file or directory
    3 | #include "header.hpp"
      |          ^~~~~~~~~~~
compilation terminated.
Error: failed to build package `current-project`

Caused by:
    internal build system has been stopped with an error:
subcommand failed