pip fails when project requirement result binary already exists

[David Parrish]

When trying to Snap the JoinMarket project on Github. I noticed that when running snapcraft for the second time, would fail because a result binary already exists, and pip didn't have a `--exists-action` set.

In the patch, I'm not sure if it would be better to (i)gnore or (w)ipe for the default action.

I've included a patch which fixes the issue for me. I also have included the error message I am getting, along with my snapcraft.yml.

Error message:

```
Updating pull step for joinmarket (source changed)
Obtaining file:///root/project/jmbase (from -r /root/parts/joinmarket/build/requirements/base.txt (line 1))
The file /root/parts/joinmarket/python-packages/joinmarketbase-0.6.1.zip exists. (i)gnore, (w)ipe, (b)ackup, (a)bort ERROR: Exception:
Traceback (most recent call last):
  File "/root/parts/joinmarket/install/lib/python3.6/site-packages/pip/_internal/cli/base_command.py", line 186, in _main
    status = self.run(options, args)
  File "/root/parts/joinmarket/install/lib/python3.6/site-packages/pip/_internal/commands/download.py", line 135, in run
    resolver.resolve(requirement_set)
  File "/root/parts/joinmarket/install/lib/python3.6/site-packages/pip/_internal/legacy_resolve.py", line 177, in resolve
    discovered_reqs.extend(self._resolve_one(requirement_set, req))
  File "/root/parts/joinmarket/install/lib/python3.6/site-packages/pip/_internal/legacy_resolve.py", line 333, in _resolve_one
    abstract_dist = self._get_abstract_dist_for(req_to_install)
  File "/root/parts/joinmarket/install/lib/python3.6/site-packages/pip/_internal/legacy_resolve.py", line 265, in _get_abstract_dist_for
    return self.preparer.prepare_editable_requirement(req)
  File "/root/parts/joinmarket/install/lib/python3.6/site-packages/pip/_internal/operations/prepare.py", line 559, in prepare_editable_requirement
    req.archive(self.download_dir)
  File "/root/parts/joinmarket/install/lib/python3.6/site-packages/pip/_internal/req/req_install.py", line 721, in archive
    display_path(archive_path), ('i', 'w', 'b', 'a'))
  File "/root/parts/joinmarket/install/lib/python3.6/site-packages/pip/_internal/utils/misc.py", line 223, in ask_path_exists
    return ask(message, options)
  File "/root/parts/joinmarket/install/lib/python3.6/site-packages/pip/_internal/utils/misc.py", line 241, in ask
    response = input(message)
EOFError: EOF when reading a line
Failed to run '/root/stage/usr/bin/python3 -m pip download --disable-pip-version-check --dest /root/parts/joinmarket/python-packages --requirement /root/parts/joinmarket/build/requirements/gui.txt': Exited with code 2.

```

snapcraft.yaml

```
name: joinmarket
base: core18
version: master
summary: Software to assist in creating bitcoin CoinJoin transactions
description: |
  JoinMarket is software to create a special kind of bitcoin transaction called a CoinJoin transaction. It's aim is to improve the confidentiality and privacy of bitcoin transactions.
confinement: devmode
grade: devel
#icon: icon.png

apps:
  joinmarket-qt:
    extensions:
      - kde-neon
    command: usr/bin/jm
    plugs:
      - network
      - network-bind
      - desktop
      - desktop-legacy
      - x11
      - wayland
      - home

parts:
  joinmarket:
    plugin: python
    source: .
    requirements:
      - requirements/gui.txt
    override-build: |
      snapcraftctl build

      pwd
      ls -alh
      #cd ../install
      mkdir -p /root/parts/joinmarket/install/opt
      cp -rp /root/parts/joinmarket/build /root/parts/joinmarket/install/opt
      mv /root/parts/joinmarket/install/opt/build /root/parts/joinmarket/install/opt/joinmarket

  # Install desktop launcher
  launchers:
    plugin: dump
    source: .
    organize:
      'snap/jm': usr/bin/jm
    after:
      - joinmarket
```