Switch to packages module, allow to specify mod folder

[ale-rinaldi]

Fix #3

Use the golang.org/x/tools/go/packages module instead of the build one. Add an optional mod parameter to specify the vendor directory.

[pdt256]

This PR broke finding updated dependencies for me.

[kynrai]

I think we could maybe just have a modules using this new packages lib and non modules using the old build lib with a switch between them