stage-snap causes issues when mixing cores

I came across this issue whilst trying to stage a core16 snap into a core18.

It seems that, when mixing these cores, your build environment (during snapcraft build) is overwritten by the unpacking of the incoming stage snap. This obviously breaks build packages that you have. E.g. this yaml https://www.irccloud.com/pastebin/mxelP3CN/ gives errors like:

```
Fatal Python error: Py_Initialize: Unable to get the locale encoding
ImportError: No module named 'encodings'
```

```
fatal: unable to find remote helper for 'https
```

Even if you put this stage snap to the bottom of the build process, you will end up with an error like:

```
Failed to stage: Parts 'aws-cli' and 'docker' have the following files, but with different contents:
    etc/mime.types
    lib/x86_64-linux-gnu/libexpat.so.1
    usr/bin/pdb3
    usr/bin/py3clean
    usr/bin/py3compile
    usr/bin/pydoc3
    usr/bin/pygettext3
    usr/bin/python3
    usr/bin/python3m
    usr/bin/run-mailcap
    usr/lib/valgrind/python3.supp
    usr/lib/x86_64-linux-gnu/libexpatw.so.1
    usr/lib/x86_64-linux-gnu/libffi.so.6.0.4
    usr/lib/x86_64-linux-gnu/libmpdec.so.2.4.2
    usr/lib/x86_64-linux-gnu/libsqlite3.so.0.8.6
    usr/share/doc/libexpat1/changelog.Debian.gz
    usr/share/doc/libexpat1/copyright
    usr/share/doc/libffi6/changelog.Debian.gz
    usr/share/doc/libmpdec2/changelog.Debian.gz
    usr/share/doc/libpython3-stdlib/README.Debian
    usr/share/doc/libpython3-stdlib/changelog.Debian.gz
    usr/share/doc/libsqlite3-0/changelog.Debian.gz
    usr/share/doc/mime-support/changelog.gz
    usr/share/doc/python3-minimal/README.Debian
    usr/share/doc/python3-minimal/changelog.Debian.gz
    usr/share/man/man1/pdb3.1.gz
    usr/share/man/man1/pydoc3.1.gz
    usr/share/man/man1/pygettext3.1.gz
    usr/share/man/man1/python3.1.gz
    usr/share/man/man1/python3m.1.gz
    usr/share/man/man1/run-mailcap.1.gz
    usr/share/pixmaps/python3.xpm
    usr/share/python3/debian_defaults
    usr/share/python3/debpython/version.py
    usr/share/python3/py3versions.py

Snapcraft offers some capabilities to solve this by use of the following keywords:
    - `filesets`
    - `stage`
    - `snap`
    - `organize`

To learn more about these part keywords, run `snapcraft help plugins`.
```

I propose that snapcraft gives a warning when pulling in the snap to be staged if the cores mismatch.