Alibuild

We now force environment-modules to run in 3.x compatible mode to prevent further breaks. In particular, a critical problem with the new `modulecmd` aborting when a dependency is not found was fixed (as our GCC-Toolchain selection mechanism is based on that, for instance).

* Read-only `MIRROR` directory is now handled correctly
* Added proper documentation and troubleshooting
* General optimization of tests related to this feature

Bug fixes:

* Checking for system deps resulted in inconsistent exit codes: force use of /bin/bash when checking system deps
* Running aliBuild was slow and randomly returned errors at the initial fetching phase: do it only when needed, with a switch to force-enable it
* Homebrew-provided environment-modules is now at version 4, breaking alienv: alienv now supports it fully

General improvements:

* Add Fedora to the list of supported platforms
* Show build percentage in non-debug mode (guess it from stdout, useful for CMake-based projects)

* Refactor code and support for the pb multicommand
* Improve arch detection in alienv
* Add proper Unicode support for both Python 2 and 3
* Fix bug causing undesired rebuilds on Python 3
* Keep recipe ordering of environment variables
* Add proper relocation on macOS
* Improve support for defaults

* Refactor code and support for the `pb` multicommand
* Improve arch detection in `alienv`
* Add proper Unicode support for both Python 2 and 3
* Fix bug causing undesired rebuilds on Python 3
* Keep recipe ordering of environment variables
* Add proper relocation on macOS
* Improve support for defaults

No need to sudo on macOS.

People using the Git repository directory might have to install PyYAML now, since aliBuild does not use it's own copy to guarantee compatibility with Python 3.