python shebang rewriting can fail in presence of a stage-package

[Jonathan Cave]

Attempt to snap the following yaml:

---
name: python-with-two-parts-stage-packages
version: 0
summary: test the python plugin
description: |
  This is a snap with two python parts and stage-packages
confinement: strict
grade: devel

parts:
  part1:
    source: .
    plugin: python
    stage-packages:
      - python3-yaml
  part2:
    source: .
    plugin: python
    python-packages:
      - bitstring
    after: [part1]
---

Note that after the build step of `part1` some files will have a shebang line (on xenial here):
#! /usr/bin/python3.5

This will prevent staging of `part2` whose shebang lines appear to modified to the desired:
#!/usr/bin/env python3.5

[Kyle Fazzari (kyrofa)]

I propose fixing all python shebangs in stage-packages rather than just the bin directories. This proposal is available in code form here: https://github.com/snapcore/snapcraft/pull/2112