cmd/go/internal/modget: factor out functions for argument resolution

For #37438
For #41315
For #36460

Change-Id: I17041c35ec91ff6ffb547e0f32572673d191b1ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/254820
Trust: Bryan C. Mills <[email protected]>
Trust: Jay Conrod <[email protected]>
Run-TryBot: Bryan C. Mills <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>
Reviewed-by: Jay Conrod <[email protected]>

Author: Bryan C. Mills