Use goimports for import grouping

[enj]

I have this mostly automated:

goimports does stdlib, 3rd-party, and then local.

I have goland set to run goimports -local github.com/Azure/azure-workload-identity -w <filename> on save.

// LocalPrefix is a comma-separated string of import path prefixes, which, if
// set, instructs Process to sort the import paths with the given prefixes
// into another group after 3rd-party packages.
var LocalPrefix string

Originally posted by @enj in #722 (comment)