x/tools/cmd/goimports: adds circular imports

[stamblerre]

goimports will add an import for the package you are currently in if you make a mistake.

Repro:

Open a file in golang.org/x/tools/internal/lsp/source and replace "View" with "source.View". An import for golang.org/x/tools/internal/lsp/source will be added by goimports.

[gopherbot]

Change https://golang.org/cl/170238 mentions this issue: imports: fix circular imports