Allow relative paths in the 'remotePath' for a launch.json

[rchiodo]

Partial fix for #1301

Users in issue 1301 were attempting to do something like so:

"pathMappings": [
                {
                    "localRoot": "${workspaceFolder}",
                    "remoteRoot": "." // <-- '.' is not supported here.
                }
            ]

In pydevd this isn't supported. The remote and local roots have to be fully specified. This is a pain in a shared situation though as you can pass environment variables for the local path but not the remote.

This change allows the user to have a relative (to the current working directory) for the remote path.

[AdamYoblick]

This is crazy to read 😆