move Logger into Ctx; restore logging

[jmank88]

This change:

• moves the Loggers definition alongside Ctx
• adds a Loggers field to Ctx
• restores warning logging

Fixes #531

[darkowlzz]

oops! #545 😅

[sdboyer]

I was hasty and incorrect when laying out the desired goal here in that FIXME. Sorry, I need to backtrack a bit.

We shouldn't be logging anything from within readManifest(); that decision needs to be made at a higher level.

1. Instead, if any warnings are encountered, we should pass those out as well, via a third return value (so the sig becomes func readManifest(r io.Reader) (*Manifest, []error, error)).
2. When calling readManifest() from Analyzer.DeriveManifestAndLock(), for the moment, we should ignore the new []error return value.
3. When calling readManifest() from Ctx.LoadProject(), if there are warnings returned, we should immediately log them to stderr, but continue.

The basic idea here is that, when the Analyzer is reading a manifest, it's doing so for a dependency on behalf of the SourceManager. It's not examining the current project. In that case, we want to be lenient, as those errors are in other projects' Gopkg.toml - something the user is unlikely to be able to do anything about. Showing non-actionable errors to the user is generally a mistake.

When Ctx is reading Gopkg.toml, though, it's doing it for the current project. The user IS empowered to do something about that. So we show the errors.

I'm tempted to make #3 even stronger and just immediately exit nonzero with an appropriate general error - e.g. "Invalid values found in Gopkg.toml". But that's quite draconian, and out of keeping with how most tools treat their config files. Plus, it would make it very awkward for us to add new fields to Gopkg.toml in the future, which is something we definitely want to do. So we'll have to accept just warnings.

[jmank88]

we should the new []error return value.

I think you accidentally a word. Though the following paragraph suggests 'ignore', is that correct?

[sdboyer]

I think you accidentally a word. Though the following paragraph suggests 'ignore', is that correct?

And such a crucial word, lol - yes, "ignore" was what I dropped. (editing now)

[sdboyer]

brilliant - thank you!