does not catch replace / forks

[gyakovlev]

I haven't looked how it works, but I noticed that it does not detect replace directive from go.mod

for example: aerc-3.0 go.mod file has the following line:

replace github.com/gdamore/tcell => git.sr.ht/~sircmpwn/tcell v0.0.0-20190807054800-3fdb6bc01a50  

yet get-ego-vendor generates this "github.com/gdamore/tcell 84b54971b46c" as a vendor string, but really it should be "github.com/gdamore/tcell 3fdb6bc01a50 git.sr.ht/~sircmpwn/tcell"

This allows eclass to download forked snapshot ot tcell, as upstream intended.

[williamh]

The current version of this tool uses go.sum instead of go.mod, so we should be good now.
Take a look and let me know.