Refactor solution imports to provide PS native progress indication

[matwilko]

Fixes #480.

I should note that this still needs some testing, and I'll be using my org as guinea pigs over the next week or so to work out any kinks, but wanted to get this in front of you upfront for any comments on approach/obvious problems.

[seanmcne]

Awesome! Please let me know how this pans out, both in interactive testing but if you get the chance to also experiment with headless operations (this is used quite a bit via AzureDevOps for build/release pipelines). Thanks again for your effort & contribution! Shoot me a quick teams message or heads up when you think you've been through a bit more testing and verification.

[matwilko]

Good point, I was optimizing for our developer inner-loop here - I'll add a helper method to output both Write-Progress and Write-Verbose so headless gets equivalent feedback :)

[matwilko]

Apologies that this is languishing, I've had some trouble getting this merged into use internally (yay priorities!) so don't have a wider audience of testers hammering this just yet. Will let you know when it's been battle tested a little :)

[matwilko]

So this has now been hammered a bit internally, and after fixing some small bugs around checking completion status, this is now working well on a daily basis :)

As requested, I've added Write-Verbose alongside the Write-Progress's to write out progress in headless scenarios.