Part's without `source` key has an implicit source `.`

[林博仁(Buo-ren, Lin)]

Consider this snapcraft parts recipe:

```yaml
  ccache:
    plugin: nil
    build-packages:
      - ccache
    override-build: 'true'
    override-stage: 'true'
    override-prime: 'true'
```

And according to the description of `snapcraft help nil`:

> The nil plugin is useful for parts with no source.

However after running `snapcraft pull ccahe`, non-snapcraft-generated files in the source tree are pulled into the part's sourcedir:

```
parts/ccache/src
├── guvcview-brlin_source.tar.bz2
├── guvcview.strace.log
├── icons
│ ├── guvcview256.png
│ └── guvcview.png
├── LICENSE
├── nohup.out
├── README.md
└── screenshots
    └── view-image-control.png

2 directories, 8 files
```

IMO the behavior is unexpected and should be a snapcraft bug.

## Workaround
Override the entire pull step using `override-pull` [scriptlets](https://forum.snapcraft.io/t/scriptlets/4892).

[林博仁(Buo-ren, Lin) (buo-ren-lin)]

Related Snapcraft Forum topic:
[Snapcraft implicit source - snapcraft - snapcraft.io](https://forum.snapcraft.io/t/snapcraft-implicit-source/7060)

[Sergio Schvezov (sergiusens)]

If using a base, this will be the default behavior.
PR here https://github.com/snapcore/snapcraft/pull/2326