Add logging of conflicts during resolution

[stendavid]

If there are conflicts, it is difficult to see why Paket discards certain versions of a package. This patch adds some logging of conflicts during resolution to help trouble-shooting of dependencies.

Output looks like this (from the test "shold solve strange graph")
Resolving packages for group Main:

• P7 11.10.10.3
• P3 5.5.7.9
• P1 10.11.11
  Incompatible dependency: P7 <= 4.2.11.10 conflicts with resolved version 11.10.10.3
  Can't take package P1 10.11.11: incompatible dependencies
  Failed to satisfy P1 <= 10.11.11 (from P3 5.5.7.9)
• P3 1.1.3
  Failed to satisfy P8 <= 0.2.8 (from P3 1.1.3)
  Failed to satisfy P3 (from )
• P7 10.3.5.7
  Incompatible dependency: P7 <= 4.2.11.10 conflicts with resolved version 10.3.5.7
  Can't take package P1 10.11.11: incompatible dependencies
  Failed to satisfy P1 <= 10.11.11 (from P3 5.5.7.9)
  Failed to satisfy P3 (from )
• P7 4.2.11.10

For real projects with dependency problems there may be a lot of extra output, so maybe it should log only if --verbose. On the other hand, in that case it's really useful info...

[matthid]

Wow thanks wanted to add something similar. So I'll happily review this as soon as I can (probably in afew days)

[matthid]

Oh this didn't work as intended :/ I rebased your branch to trigger the CI again and GitHub doesn't seem to be able to handle that... Can you open the PR again please :)

[matthid]

Sorry @zzDavid

[matthid]

Oh now I know what I did wrong. I apparently pushed the wrong branch to your repro (which is allowed as the PR was open). But now the PR was closed by GitHub and I cannot fix it. But as you don't have the changes on your fork anymore I pushed them here: https://github.com/fsprojects/Paket/tree/zzdavid-master
I will open a PR (such that it is visible and reviewable). But I'd close that in favor of your PR if you find the time.
Sorry for the mess and confusion... Until we have a proper PR: #2379

[stendavid]

Right :) I'll fix my branch.

Also, I found out that logging without --verbose is not a good idea, at least not everything. The test "#1174 Should find Ninject error" really explodes with 60 000 log lines. (And it's still difficult to say why packages are rejected...)

[matthid]

@zzDavid We already merged and released that for testing already ;)
Will let you know when I tacke that again and have any feedback (or change the feature).

Yeah we currently have the problem that verbose is not really usable... But we need to improve on that.