Ability to specify default location of new files

[ImJohnMDaniel]

Is your feature request related to a problem? Please describe.
Currently, when you create a new file, metadata, flow, etc in a scratch org and then execute the force:source:pull command, the files automatically show up in the <projectDirectory--default:true>/main/default folder. The concept of "default" doesn't always make sense and I would like control over it.

What are you trying to do
I want a way to adjust the default location directory pattern from "/main/default" to something else

Describe the solution you'd like
Probably the simplest approach would be to either add a new environment variable to override the default location or allow a new attribute in the pacakgeDirectory section of the sfdx-project.json to allow an override.

Describe alternatives you've considered
Right now there is no alternative.

Additional context

[mshanemc]

Another alternative: if you know what you're pulling, you can use source:retrieve with --tracksource (it now lets you specify the output location).