Add download-only basic option to only download
sources and extensions without build software.
Move fetch_extension_source in fetch_step, to allow
use of --stop=fetch to only download sources and
extensions archives.
Add missing check for exts before copying in exts_all.
Add fetch argument to extensions_step for testing purpose.
Merge pull request #978 from mesocentrefc/download_only

fetch extension sources in fetch_step, enhances --stop=fetch