Import of github.com/umputun/reproxy/lib adds github.com/umputun/reproxy to go.mod. It results in a significant number of dependencies.

At the same time, github.com/umputun/reproxy/lib requires only 3 dependencies:

• github.com/go-pkgz/lgr
• github.com/go-pkgz/repeater
• github.com/stretchr/testify

Making github.com/umputun/reproxy/lib a submodule can resolve this issue.

However, this change will also require some additional work:

• manage 2 versions: vX.Y.Z for github.com/umputun/reproxy and lib/vX.Y.Z for github.com/umputun/reproxy/lib
• github.com/umputun/reproxy/lib will have to be vendored (see cmd/go: allow 'go mod vendor' to omit replaced modules golang/go#33789 and build: no longer use vendor directory hashicorp/nomad#10898)
• users of github.com/umputun/reproxy/lib will have to update dependency path and version

I don't think these points are very critical: API of github.com/umputun/reproxy/lib is pretty stable, and the current version (v0.10.0) allows to make breaking changes