fix broken rebar build

In commit cb493908 I broke the ability for yaws to be used as a rebar
dependency for other projects. Move src/yaws.app.src.in back to
src/yaws.app.src. Move the code in rebar.config.script to
src/yaws.app.src.script file to process the app dependencies for
src/yaws.app.src. Change src/Makefile to process app dependencies when
creating ebin/yaws.app.