Rename directory to avoid confusion.

runtime will contain runtime functions of basher, while lib might
contain other parts of the module.
Reorganize directories.

modules path became cellar. It will be used for modules and to have only
one path with linked binaries.
Add cellar bin to path.
Add basher-uninstall command.
Rename directories for testing.
Replace mocking functions by a generic version.
Rename test.
Create bins as part of install.