add go.mod

[thaJeztah]

looks like https://github.com/LK4D4/vndr/pull/97 doesn't like to be installed with the local source if no go module is available; guess it doesn't hurt to have one

[thaJeztah]

Ahm.. I guess that doesn't work in testing;

=== RUN   TestVndrInit
vndr_test.go:141: install github.com/LK4D4/dumbproject failed: exit status 1, out: go: finding module for package github.com/LK4D4/dumbproject
cannot find module providing package github.com/LK4D4/dumbproject: module github.com/LK4D4/dumbproject: git ls-remote -q origin in /tmp/test-vndr-375925155/pkg/mod/cache/vcs/ede4bc18a52cafd9d0dbd7b147c3ce0820fc4f1b8cb11784bca500b0375225d4: exit status 128:
fatal: could not read Username for 'https://github.com': terminal prompts disabled
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