Add julz/importas linter

[SVilgelm]

Add a new linter https://github.com/julz/importas

[SVilgelm]

This PR should be merged before the integration: julz/importas#1

[ldez]

FYI with go1.16 and our test system, if you create a test file with external imports (not in the main go.mod) you will have an issue.
I meet the same problem during the update to go1.16 and sqlclosecheck, I had to remove the imports.
4d6aa3c

[SVilgelm]

we can import system modules, like import fff "fmt"