tsrc init should be stateless

[dmerejkowsky]

Problem:
Currently the command init is halfway stateful, halfway stateless. By that I mean it is smart enough to read the manifest url from the .tsrc/manifest.yml file, but it is dumb enough to blindly overwrite the group list.

This becomes annoying in the following scenario:

• There is a default group
• You run tsrc init
• A new group foo appears
• You want to use the repos from the foo group and the default group
• You must run tsrc init -g default -g foo, because running just tsrc init -g foo would make repos from the defalult groups disappear.

Solution

• Move the .tsrc/manifest.yml file in .tsrc/workspace.yml and document its format
• Make init stateless - probably emit a warning or even an error if the file exists

The new scenario then becomes:

• There is a default group
• You run tsrc init
• A new group foo appears
• You want to use the repos from the foo group and the default group
• You edit the config in .tsrc/workspace.yml with the new list of groups
• You run tsrc sync