Get build std streams without verbose output

[vidartf]

What's the problem this feature will solve?
Normally on CI for my package, I install it with pip install -v -e ., using the verbose flag in order to get the log/stdout from my build process. Having the build output available is very useful for when the CI fails for some reason while the build itself does not. However, the verbose flag of pip also includes a lot of other outputs, filling the CI log with typically thousands of lines. What I would like to see is a way to just get the build output, without anything extra from the dependency installation.

Describe the solution you'd like
Conceptually, the simplest solution would be to add a flag for having pip pass through the build output streams. I do not know enough about the internals of pip to say how easy this is to implement though.

Then one would instead call pip install --new-flag -e .

Alternative Solutions

There might be several other ways to do this, but here are a few possibilities I thought of:

1. If pip had a --only-deps flag, one could do:

   pip install --only-deps . && pip install --no-deps -v -e .

2. Change verbosity levels to add a step in between. I'm not sure if the mapping between verbosity level and included output is considered stable/fixed. If it is not, then maybe the first verbosity level could be a little less verbose?

3. Allow CLI options and/or config files for configuring the pip loggers. Since pip cannot be called in-process any more, configuration of the loggers via the python API is now also off-limits. This would be a more general solution, where customizing verbosity would be one of several uses.

4. When calling pip, one could use the --log option or pipe the output through a filterer that scans for known markers like Running setup.py develop for <package name>. That would require that the specific text of the output could be considered stable though, which I assume it cannot currently?

[cjerdonek]

See the related #5900.

[QuLogic]

Dupe of #5531, I think.

[vidartf]

@QuLogic Thanks, closing as duplicate!