go/build: use the main module's root when locating module sources

go/build: use the main module's root when locating module sources

Previously, we were using srcDir, which would apply the wrong module
dependencies (including the wrong 'replace' and 'exclude' directives)
when locating an import path within a module.

Fixes #34860

Change-Id: Ie59dcc2075a7b51ba40f7cd2f62dae27bf58c9b0
Reviewed-on: https://go-review.googlesource.com/c/go/+/203820
Reviewed-by: Jay Conrod <jayconrod@google.com>