Use launch.json to start and debug processes

[doudou]

This handles C++ and Ruby debug configurations in launch.json to allow running them within a Rock workspace transparently.

Behind the scenes, the extension now starts all the debugged processes using the workspace's environment (using autoproj exec)

In addition, the extension provides four variable expansions that can be used within launch.json in the program and cwd fields of the debug configurations:

• ${rock:srcDir} expands to the package's source directory
• ${rock:buildDir} expands to the package's build directory
• ${rock:prefixDir} expands to the package's prefix directory
• ${rock:which:_cmd_} expands to the result of autoproj which _cmd_ as run within the workspace, i.e. solves the full path to _cmd_ with the PATH of the workspace.

For instance, a Ruby rake test configuration would be:

{
"name": "rock - Run test suite (Rake)",
"type": "Ruby",
"request": "launch",
"cwd": "${workspaceRoot}",
"program": "${rock:which:rake}",
"args": ["test"]
}

a C++ Run Rock's default test suite would be:

        {
            "name": "(gdb) Launch",
            "type": "cppdbg",
            "request": "launch",
            "program": "${rock:buildDir}/test/suite",
            "cwd": "${rock:buildDir}",
            "args": [],
            "stopAtEntry": false,
            "environment": [],
            "externalConsole": true,
            "MIMode": "gdb",
            "setupCommands": [
                {
                    "description": "Enable pretty-printing for gdb",
                    "text": "-enable-pretty-printing",
                    "ignoreFailures": true
                }
            ]
        }

[doudou]

STATUS SO FAR

• this requires unreleased work on Autoproj (for the autoproj which subcommand).
• C++ and Ruby launch tasks work, without the need to pre-source env.sh.

One provides a program path relative to the build, source or prefix dir. The extension resolves it. I think I'll change it to rather expand variables like `${autoproj:buildDir} instead, which would make it explicit.