Allow specifying location of snapcraft.yaml file

[Alberto Mardegan]

I registered a project containing snap recipes for several upstream project which I don't control. I wanted to have snap packages automatically created for them, but the "Create snap package" option is missing:

https://code.launchpad.net/~mardy/snap-recipes/+git/snaps

I think it's because this project doesn't have a snapcraft.yaml file in its toplevel directory. While I understand that a single project carrying many snapcraft recipes might not be the intended target of the snap building feature in launchpad, there are other more legitimate cases where the snapcraft.yaml won't be found in the toplevel directory.
For instance, all upstream projects to whom I contributed snapcraft recipes don't want to have them in the main project directory, but under a subdirectory (like "dist"):

https://github.com/simonfuhrmann/mve/pull/299
https://github.com/openMVG/openMVG/pull/776
https://github.com/nextcloud/client_theming/pull/77/files

So, I'd suggest that the "create snap package" page should have a field where one can specify from which directory the "snapcraft" command has to be lanched (especially now that it supports both "snapcraft.yaml" and "snap/snapcraft.yaml").