dig.In/Out: Allow and ignore unexported fields

[abhinav]

dig.In and dig.Out structs explicitly disallow unexported fields on their
structs.

dig/param.go

Line 349 in 4ea2499

"unexported fields not allowed in dig.In, did you mean to export %q (%v)?",

Based on anecdotal evidence from our internal support room, this check makes
sense because otherwise users will assume that those unexported fields will
also be filled by Fx.

However, there's value in allowing unexported fields on dig.In tagged structs
regardless, e.g. uber-go/fx#673. We may want to
support an opt-in means of specifying that a provided struct MAY have
unexported fields.

One means of doing this is struct tags on the embedded dig.In field:

type foo struct {
  dig.In `allow-unexported:"true"`  // naming TBD
  
  Foo Foo
  bar bar // ignored by Dig
}

Internal issue reference: GO-374