Use go.mod to discover import path

[twaugh]

Each go.mod file in the tree defines a separately-versioned project. We should:

• search recursively for go.mod files not in vendor directories or directories which themselves contain a go.mod file
• if any are found, use this as the list of projects (instead of searching each subdirectory of the provided dir)
• for each found go.mod, extract the module name from it to use as the import path