remote-build: support for snap relative sources

[Guillaume Beuzeboc]

My part is:

```
parts:
  ros2-galactic-extension:
    build-packages: [ros-galactic-ros-core]
    override-build: install -D -m 0755 launch ${SNAPCRAFT_PART_INSTALL}/snap/command-chain/ros2-launch
    plugin: nil
    source: $SNAPCRAFT_EXTENSIONS_DIR/ros2
```
It uses source from snapcraft. This works fine locally but When I try remote-build, I get:
```
Failed to pull source: unable to determine source type of '$SNAPCRAFT_EXTENSIONS_DIR/ros2'.
Check that the URL is correct, or consider specifying `source-type` for this part. See `snapcraft help sources` for more information.
```
I couldn't find `source-type` that would work for my case.