Follow XDG Base Directory Specification

[Lord-Valen]

To comply with XDG Base Directory Specification, the default location for DATA_DIR should be "$XDG_DATA_HOME"/atomic-swap/{ENV} and XDG_DATA_HOME should fallback to "$HOME"/.local/share. Some of the contents of DATA_DIR could also belong in XDG_STATE_HOME, XDG_CACHE_HOME, or XDG_CONFIG_HOME. The full specification can be found here.