New Manifest.toml format: Pt 2. Default to new format, warn if old format is being maintained

[IanButterworth]

New Manifest.toml format: Pt 2. (For 1.7 only, not to be backported to 1.6.2)

• Defaults to the new manifest format
• Warns the user (once) if an old format manifest is being maintained

Continuation after part 1 (#2561)

Fixes #1458
Fixes #2508
Closes #2557

[KristofferC]

One problem with deleting and running resolve is that you lose all the information in the manifest. Alternatively, we could have a very simple function Pkg.upgrade_manifest() and suggest that, which would keep all the dependency versions.

[IanButterworth]

Alternatively, we could have a very simple function Pkg.upgrade_manifest() and suggest that, which would keep all the dependency versions.

Nice. Done!

[DilumAluthge]

@StefanKarpinski @KristofferC Is this PR good to merge once CI turns green?

[KristofferC]

I made it so that the upgrade manifest function also set the version field (I think that was omitted by accident). LGTM now. We will see if the warning gets too annoying but let's just focus on getting this in right now.

[IanButterworth]

I made it so that the upgrade manifest function also set the version field (I think that was omitted by accident)

@KristofferC it was intentional. I should've added a comment. I think the version needs to represent which julia version the resolve happened in, and given the tool just updates the format I don't think it's accurate to set the version field.

[KristofferC]

That is true, let's take that back then :)

[KristofferC]

#2599