go/build, internal/dag: lift DAG parser into an internal package

This lifts the DAG parser from the go/build dependencies test into its
own package that can be reused elsewhere.

I tried to keep the code as close as possible. I changed some names to
reflect the more general purpose of internal/dag. Most of the changes
are related to error handling, since internal/dag doesn't take a
testing.T on which to report errors. Notably, parseRules now returns a
slice of parsed rules rather than calling a callback because this made
it easier to separate fatal parsing errors from non-fatal graph
checking errors.

For #53789.

Change-Id: I170b84fd85f971cfc1a50972156d48e78b45fce3
Reviewed-on: https://go-review.googlesource.com/c/go/+/418592


Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>