unixfs: Use only one kind of variable to refer to data types

[schomatis]

There are a series of alias for the protocol buffer types of the Data message (format), in some parts of the code the alias are used (e.g., TFile) while in others the original variables are (e.g., pb.Data_File) which makes it harder to follow the code logic, these should be unified and only one kind of variable should be used.

https://github.com/ipfs/go-ipfs/blob/e235d02188260a7a3df18209df1f11b6bf561483/unixfs/unixfs.go#L15-L19

https://github.com/ipfs/go-ipfs/blob/e235d02188260a7a3df18209df1f11b6bf561483/importer/helpers/dagbuilder.go#L119

https://github.com/ipfs/go-ipfs/blob/e235d02188260a7a3df18209df1f11b6bf561483/mfs/dir.go#L168

(Also the unixfs and the unixfs/pb have several different import alias which makes it even harder to quickly infer from the code what are the types referring to without having to look at the import statements.)

I would be inclined to remove the alias (to eliminate the possibitlity of this double reference) and simplyfiy names at the protocol buffer level in the .proto file (if possible, that will be discussed in a separate issue).

[schomatis]

At this point I don't think we can rename the proto-buf names, so let's try to use the unixfs short alias as much as possible throughout the code base (and thus avoid direct reference to the unixfs_pb package -inunixfs.pb.go- as much as possible as we've been doing with ipfs/go-unixfs#17).

/cc @overbool

[overbool]

@schomatis Did you mean that we should use data type under unixfs package instead of data type under unixfs_pb package as much as possible throughout the code base , eg, using unixfs.TFile instead of pb.Data_File?

[schomatis]

Yes exactly!