cmd/eg: use go/packages
cmd/eg: use go/packages

Rewrite eg to use go/packages, so that it works with Go modules.

There are two known regressions:

* lost build tag support
* you can't use a template to add a new dependency

These can be re-added at a future point as needed.

Change-Id: I2b59bb5e101c738a4d4c40e84a12c5b3a19bcd18
Reviewed-on: https://go-review.googlesource.com/c/tools/+/258304
Trust: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools