fix(namesys): handling of multiple DNSLink TXT records

[hacdias]

This addresses #507. I made individual commits to be easy to follow.

I removed the legacy support for DNSLink entries at the root of the domain. This made it easier to plug in the errors from the DNS. My issue was that if both rootCh and subCh returned an error, and the error was different, which one to choose? Probably subCh, but that still felt fragile. I can maintain legacy support if we want it, but that will result in a bit more work.

I also added two new errors to the package, to make it easier to test against specific types of errors.

[codecov]

Codecov Report

Merging #510 (d277609) into main (49e0f39) will decrease coverage by 0.04%.
The diff coverage is 80.55%.

@@            Coverage Diff             @@
##             main     #510      +/-   ##
==========================================
- Coverage   65.52%   65.49%   -0.04%     
==========================================
  Files         207      204       -3     
  Lines       25553    25469      -84     
==========================================
- Hits        16743    16680      -63     
+ Misses       7336     7319      -17     
+ Partials     1474     1470       -4     

Files	Coverage Δ
namesys/interface.go	74.46% <ø> (ø)
namesys/dns_resolver.go	72.17% <80.55%> (-5.17%)	⬇️

... and 13 files with indirect coverage changes

[lidel]

LGTM. Removing undocumented legacy lookup is ok. People had multiple years to migrate to _dnslink, and the error message will be clear:

Error: DNSLink lookup for "google.com." failed: could not resolve name: DNSLink lookup could not find a TXT record (https://docs.ipfs.tech/concepts/dnslink/)

Added positive test for valid duplicates that we don't care about (making sure that is ignored correctly – we don't want to gatekeep DNSLink, people should be able to use it for non-IPFS namespaces if they want)

Tested it end-to-end against latest Kubo and works as expected:

$ dig +short TXT _dnslink.invalid-duplicate-dnslink-test.example.com.
"dnslink=/ipfs/QmbWqxBEKC3P8tqsKc98xmWNzrzDtRLMiMPL8wBuTGsMnR"
"dnslink=/ipfs/bafybeigdyrzt5sfp7udm7hu76uh7y26nf3efuylqabf3oclgtqy55fbzdi"

$ ipfs resolve /ipns/invalid-duplicate-dnslink-test.example.com
Error: DNSLink lookup for "invalid-duplicate-dnslink-test.example.com." failed: could not resolve name: DNSLink lookup returned more than one IPFS content path; ask domain owner to remove duplicate TXT records

$ dig +short TXT _dnslink.valid-duplicate-dnslink-test.example.com.
"dnslink=/foo/bar"
"dnslink=/ipfs/bafybeigdyrzt5sfp7udm7hu76uh7y26nf3efuylqabf3oclgtqy55fbzdi"

$ ipfs resolve /ipns/valid-duplicate-dnslink-test.example.com
/ipfs/bafybeigdyrzt5sfp7udm7hu76uh7y26nf3efuylqabf3oclgtqy55fbzdi