When no source URL defined, the following error occurs:
/home/builder/termux-packages/scripts/build/get_source/termux_step_get_source.sh: line 2: TERMUX_PKG_SRCURL: unbound variable
Calls termux_git_clone_src if TERMUX_PKG_SRCURL ends with .git, and
termux_download_src_archive and termux_extract_src_archive otherwise.
termux_step_extract_package has been split up into the latter two
functions.
termux_step_post_extract_package has been renamed to
termux_step_post_get_source to reflect these changes.