Req: Warn and stop install if pip is about to clobber over existing files

[bobpaul]

• Pip version: 9.0.1
• Python version: 3.6.4
• Operating system: Linux 4.14.12 on ArchLinux

Description:

Recently I found pyserial and serial packages install to the same paths. While this is outside pip's control/scope, it seems like pip should at least halt installation if it sees files are about to be clobbered.

What I've run:

# pip install serial
... no errors or warnings...
$ python -c 'import serial; print(serial.__doc__, serial.__file__)'
``serial`` is an object serialization/deserialization library intended to facilitate authoring of API models which are
readable and introspective, and to expedite code and data validation and testing. ``serial`` supports JSON, YAML, and
XML.
 /usr/lib/python3.6/site-packages/serial/__init__.py
# pip install pyserial
... no errors or warnings ...
$ python -c 'import serial; print(serial.__doc__, serial.__file__)'
None /usr/lib/python3.6/site-packages/serial/__init__.py
# pip uninstall pyserial
$ python -c 'import serial; print(serial.__doc__, serial.__file__)'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
AttributeError: module 'serial' has no attribute '__file__'

What I expected

During pip install pyserial I expected pip to warn me that /usr/lib/python3.6/site-packages/serial/__init__.py and other files already existed.

[pradyunsg]

Duplicate of #4625