rename dagpb protobuf to merkledag.v1

[aschmahmann]

This prevents collisions with the go-merkledag protobuf names. Similar to #318 and #212 and part of resolving #291.

Note: unlike with #318 the protobufs here cannot be regenerated without changing the hashes of tooling like ipfs add <file>. Regeneration would also likely result in a violation of the dag-pb spec since field 2 is supposed to come before 1 even though spec violations occur in the wild and implementations should parse even if 1 comes before 2.

We should test this in kubo to make sure all those test fixtures pass, but any 👀 on if there are more manual changes to be made here would be appreciated (e.g. the .proto file and the file descriptor are left untouched when compared with the ipld/unixfs PR).

Note: this was tested in ipfs/kubo#9907 which is passing.

[codecov]

Codecov Report

Merging #323 (a0b3155) into main (f29903b) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #323      +/-   ##
==========================================
- Coverage   49.59%   49.56%   -0.03%     
==========================================
  Files         283      283              
  Lines       33873    33873              
==========================================
- Hits        16798    16789       -9     
- Misses      15314    15322       +8     
- Partials     1761     1762       +1     

Impacted Files	Coverage Δ
ipld/merkledag/pb/merkledag.pb.go	56.70% <100.00%> (+0.35%)	⬆️

... and 10 files with indirect coverage changes