Validator for the snap's icon appears to check the file before the snap is compiled

[TMiguelT]

Running `snapcraft` with the `icon` field set to something inside the container (e.g. `share/icons/hicolor/48x48/apps/myapp.png`) results in the following validation error:
`Issues while validating snapcraft.yaml: Specified icon ./share/icons/hicolor/48x48/apps/myapp.png' does not exist`

This would suggest that the icon needs to be the relative path of the icon outside the container. However the docs seem to indicate that the icon path should be a path inside the container: https://github.com/snapcore/snapcraft/blob/master/docs/metadata.md#package-icon

In addition, running `snapcraft` with a filepath outside the container means it passes this initial validation check, but results in this error at the very end of the compilation stage:

[Errno 2] No such file or directory: './myapp/resources/appname.png' -> '/home/myuser/snap/build/dir/prime/meta/gui/icon./myapp/resources/myapp'