Support nested go.mod|sum file updates with the "Update Go" workflow

[dmikusa]

Describe the Enhancement

The workflow generated to "Update Go" currently only works for projects where the go.mod|sum files are in the root of the project. It doesn't look at nested go.mod|sum files.

Possible Solution

It is possible that we could find nested files and either generate multiple workflows or generate one workflow capable of updating multiple sets of go.mod|sum files.

Motivation

This comes up in libcnb. The project has a top-level go.mod|sum file set for the library and then one that's nested for its tools. It could be helpful to update both automatically.