building packages: create termux_step_get_source function

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.

scripts/build/get_source/termux_step_get_source.sh

@@ -0,0 +1,10 @@
+termux_step_get_source() {
+	if [ "${TERMUX_PKG_SRCURL: -4}" == ".git" ]; then
+		termux_git_clone_src
+	else
+		mkdir -p $TERMUX_PKG_SRCDIR            
+		termux_download_src_archive
+		cd $TERMUX_PKG_TMPDIR
+		termux_extract_src_archive
+	fi
+}