cmd/importsort: Replace csv flag with []string
cmd/importsort: Replace csv flag with []string

For example, the following will group all goarista imports together
followed by all github.com/aristanetworks imports.

importsort -s "github.com/aristanetworks/goarista" -s "github.com/aristanetworks"

Instead of

importsort -s "github.com/aristanetworks/goarista,github.com/aristanetworks"

Change-Id: I05cc1aa12e014837ee12436966263109599937ce

aristanetworks/goarista,aristanetworks/goarista