Marking conda as installer for pip packages

All of the python package recipes which have `python -m pip install ...` or just `pip install ...` create one problem. They install a file by the name 'INSTALLER' in the dist-info directory. For example:
```
$ cat /Users/nwani/m3/envs/test/lib/python3.7/site-packages/dbf-0.97.11.dist-info/INSTALLER
pip
```
This is bad, because the from a user's perspective, when an end user does: conda install dbf this package is installed via 'conda' and not 'pip'.
The upcoming release of pip will have a new function [was_installed_by_pip()](https://github.com/pypa/pip/blob/cff2367bf53c6dce659c078572ca0caa0e5e2f00/src/pip/_internal/utils/outdated.py#L61-L72):
This is going to be used for not telling the user to `pip install --upgrade pip` if pip is not installed via pip. I was thinking of extending it to all packages not installed by pip pypa/pip#5605 .

So, what would you recommend, conda-forge? Should we:
- stop using `pip install` and use the old `--single-version-externally-managed`
- start doing `echo conda > /path/to/pkg-pkgversion.dist-info/INSTALLER`
- add a patch to pip which adds an argument to mark the installer
- delete the INSTALLER file?