Update staticcheck to v0.1.0 (2020.2)

[ldez]

The update to v0.1.0 aka 2020.2 will have an impact on the current linters:

• https://staticcheck.io/changes/2020.2

Ref: @dominikh #1568 (comment)

Let's discuss it in this issue.

[ldez]

We will have to rewrite the unused linter to use the new implementation and the CheckExported, related to LoadModeWholeProgram, will have to be dropped.

[ldez]

another important element:

In particular, all exported package-level identifiers will be considered used from now on, even if these identifiers are declared in package main or tests, even if they are otherwise unused.