python3 plugin cannot create hard link

[Curtis Hovey]

The python3 plugin fails on my xenial host during the parts phase.
tar: ./usr/bin/python3.5: Cannot hard link to ‘./usr/bin/python3.5m’:

From snapcraft.yaml
parts:
find-text:
plugin: python3
source: src

$ snapcraft clean
$ snapcraft snap
...
tar: ./usr/bin/python3.5: Cannot hard link to ‘./usr/bin/python3.5m’: Operation not permitted
tar: Exiting with failure status due to previous errors
dpkg-deb: error: subprocess tar returned error exit status 2
/home/curtis/Work/find-text/parts/find-text/ubuntu/download/python3.5-minimal_3.5.2-2~16.01_amd64.deb

$ ls /home/curtis/Work/find-text/parts/find-text/install/usr/bin/python*
python3.5m-config
python3-config
python3m
python3m-config
python3.5-config
python3.5m

This is repeatable with the dcos-cli plugin from https://github.com/ubuntu/snappy-plaype

This issue was first reported at https://github.com/ubuntu/snappy-playpen/issues/232

[Shaun Greene (ductape)]

This also shows up when running Ubuntu 16.04 in Vagrant with a Mac OS host. Since Vagrant maps ./ from the [Mac OS] present working directory, the hardlink from `./parts/${PROJECT}/ubuntu/download/python3.5-minimal_...deb` is part of the Mac OS filesystem which the VM is not allowed to hardlink to [I believe]. The error message is the exact same as above.

To recreate my environment, use a Mac OS host with Vagrant running an Ubuntu 16.04 box and follow the snapcraft tutorial for Python snaps from within the Vagrant VM: https://tutorials.ubuntu.com/tutorial/snap-a-python-application. The error shows up on the `snapcraft prime` step.