chore: merge probe-engine into probe-cli (#201)

This is how I did it:

1. `git clone https://github.com/ooni/probe-engine internal/engine`

2. ```
(cd internal/engine && git describe --tags)
v0.23.0
```

3. `nvim go.mod` (merging `go.mod` with `internal/engine/go.mod`

4. `rm -rf internal/.git internal/engine/go.{mod,sum}`

5. `git add internal/engine`

6. `find . -type f -name \*.go -exec sed -i 's@/ooni/probe-engine@/ooni/probe-cli/v3/internal/engine@g' {} \;`

7. `go build ./...` (passes)

8. `go test -race ./...` (temporary failure on RiseupVPN)

9. `go mod tidy`

10. this commit message

Once this piece of work is done, we can build a new version of `ooniprobe` that
is using `internal/engine` directly. We need to do more work to ensure all the
other functionality in `probe-engine` (e.g. making mobile packages) are still WAI.

Part of https://github.com/ooni/probe/issues/1335

internal/engine/geolocate/mmdblookup.go

@@ -0,0 +1,53 @@
+package geolocate
+
+import (
+	"net"
+
+	"github.com/oschwald/geoip2-golang"
+)
+
+type mmdbLookupper struct{}
+
+func (mmdbLookupper) LookupASN(path, ip string) (asn uint, org string, err error) {
+	asn, org = DefaultProbeASN, DefaultProbeNetworkName
+	db, err := geoip2.Open(path)
+	if err != nil {
+		return
+	}
+	defer db.Close()
+	record, err := db.ASN(net.ParseIP(ip))
+	if err != nil {
+		return
+	}
+	asn = record.AutonomousSystemNumber
+	if record.AutonomousSystemOrganization != "" {
+		org = record.AutonomousSystemOrganization
+	}
+	return
+}
+
+// LookupASN returns the ASN and the organization associated with the
+// given ip using the ASN database at path.
+func LookupASN(path, ip string) (asn uint, org string, err error) {
+	return (mmdbLookupper{}).LookupASN(path, ip)
+}
+
+func (mmdbLookupper) LookupCC(path, ip string) (cc string, err error) {
+	cc = DefaultProbeCC
+	db, err := geoip2.Open(path)
+	if err != nil {
+		return
+	}
+	defer db.Close()
+	record, err := db.Country(net.ParseIP(ip))
+	if err != nil {
+		return
+	}
+	// With MaxMind DB we used record.RegisteredCountry.IsoCode but that does
+	// not seem to work with the db-ip.com database. The record is empty, at
+	// least for my own IP address in Italy. --Simone (2020-02-25)
+	if record.Country.IsoCode != "" {
+		cc = record.Country.IsoCode
+	}
+	return
+}