Support multi-modules project

For example, my project (a git repo) has several sub folders, each contains a `go.mod`. The project root folder may or may not contain a `go.mod`.

Is that possible to run `gotestsum` to execute all tests in each modules?

Start by reviewing how gotestsum discovers packages and invokes `go test`. Check the repository's existing test-runner coverage for projects with and without a root `go.mod`, then define done as running tests across every module under the repository while preserving the existing summary and failure behavior.

Written by the indexing model from the issue text.