net/internal/dns: new package

[mikioh]

There are issues related to DNS message handling in the net package. Probably having a separate package would make sense because it would be used not only for bug fixes but adding new features such as DNSSEC+EDNS0, mDNS.

• net: add EDNS0 support to builtin DNS stub resolver #6464
• net: document and check LookupCNAME on non-CNAME hosts #8516
• net: LookupSRV and search domains when name is empty #9391
• net: provide RFC 6724 address sorting functionality #10552 (partly)
• net: target domain names in SRV records should not be compressed #10622
• net: mishandling of dotted labels in domain names #10631

[bradfitz]

I agree that a new package would probably clean up the net package. I'm not sure I agree about mDNS at least, though.

[mikioh]

Merged into #16218.