Migrate go-unixfs into unixfs/ipld-merkledag

[Jorropo]

We have multiple and will have more unixfs implementations in the future.
So I think it could make sense to start migrating them like this, so the import path tells you which abstraction you are using. (go-unixfsnode would be unixfs/ipld-prime).

Depends on (include) #58
Depends on ipfs/go-block-format#37

Todo:

• Update go.mod with a tagged release once refactor: deprecate and add stub types to go-libipfs/blocks go-block-format#37 is released.

[Jorropo]

Notes:
Do not merge now. I'll take care of reopening this once this is ready.

[codecov]

Codecov Report

Merging #36 (041e499) into main (302b279) will increase coverage by 9.04%.
The diff coverage is 63.11%.

@@            Coverage Diff             @@
##             main      #36      +/-   ##
==========================================
+ Coverage   18.50%   27.55%   +9.04%     
==========================================
  Files          96      110      +14     
  Lines       10337    12967    +2630     
==========================================
+ Hits         1913     3573    +1660     
- Misses       8152     8899     +747     
- Partials      272      495     +223     

Impacted Files	Coverage Δ
unixfs/ipld-merkledag/file/unixfile.go	0.00% <0.00%> (ø)
unixfs/ipld-merkledag/io/resolve.go	0.00% <0.00%> (ø)
unixfs/ipld-merkledag/importer/importer.go	33.33% <33.33%> (ø)
unixfs/ipld-merkledag/test/utils.go	47.82% <47.82%> (ø)
...ixfs/ipld-merkledag/importer/trickle/trickledag.go	53.58% <53.58%> (ø)
unixfs/pb/unixfs.pb.go	56.47% <56.47%> (ø)
unixfs/ipld-merkledag/mod/dagmodifier.go	57.21% <57.21%> (ø)
unixfs/ipld-merkledag/importer/balanced/builder.go	57.57% <57.57%> (ø)
...ixfs/ipld-merkledag/importer/helpers/dagbuilder.go	61.23% <61.23%> (ø)
unixfs/ipld-merkledag/io/directory.go	62.70% <62.70%> (ø)
... and 4 more

[rvagg]

I hope the use of the name "merkledag" in this doesn't mean you're intending to pull in go-merkledag .. we really need that package to be retired over time, not baked in to some official thing

[Jorropo]

@rvagg, I havn't thought about go-merkledag yet, I don't really mind migrating here and removing it a few months later.

The main goal of naming it ipld-merkledag is to leave space for other unixfs implementations in that package.

[Jorropo]

@rvagg it's on the list ipfs/kubo#8543 (comment) 🤷

[rvagg]

🤦 we're never going to kill these repos .. just shuffle them around, making breaking changes, aggravating our ecosystem; this is not a great idea. It might serve Kubo nicely, but I don't see how it serves anyone else.

[Jorropo]

Not doing that now