Put package name prefix into build output lines

[metux]

Currently, the build run output (stdout, stderr) is just written through as-is (just passing through stdout/stderr to sub processes). But for longer build pipelines it would be very helpful to see which package is currently built.

Therefore: catch the subprocess output line per line and add a proper prefix before writing them out again.

[metux]

Possibly helpful:

https://stackoverflow.com/questions/36277995/how-to-run-python-subprocess-and-stream-but-also-filter-stdout-and-stderr
https://stackoverflow.com/questions/25750468/displaying-subprocess-output-to-stdout-and-redirecting-it
https://stackoverflow.com/questions/2715847/python-read-streaming-input-from-subprocess-communicate
https://stackoverflow.com/questions/4984428/python-subprocess-get-childrens-output-to-file-and-terminal
https://stackoverflow.com/questions/17190221/subprocess-popen-cloning-stdout-and-stderr-both-to-terminal-and-variables
https://stackoverflow.com/questions/4984428/python-subprocess-get-childrens-output-to-file-and-terminal/4985080#4985080
https://stackoverflow.com/questions/17190221/subprocess-popen-cloning-stdout-and-stderr-both-to-terminal-and-variables/25960956#25960956
https://stackoverflow.com/tags/subprocess/info
https://docs.python.org/2/library/subprocess.html
https://www.endpoint.com/blog/2015/01/28/getting-realtime-output-using-python