don't put NoSourceNode into a *FileNode

[jhump]

The buf repo is using an earlier revision of protocompile, under the moniker github.com/jhump/protocompile, for formatting.

While integrating protocompile into the buf build command, I was also removing this old dependency and updating the format command to use the latest version of this repo (the one under the bufbuild organization).

While debugging some discrepancies, I encountered a bug in this repo: an empty file resulted in an AST that would cause a panic when trying to visit/walk the AST. The problem is that it would store a NoSourceNode into the empty file node -- but that is a special placeholder node that means there is no AST, so it is (intentionally) not handled by the visit/walk functionality.

This fixes it. An empty file now looks like a normal file, but with no declarations. The singular token in the AST is now just the EOF token. (The reason a NoSourceNode was previously used was just to have some singular token, because the implementation of compositeNode gets cranky if it has zero children.) I also updated the grammar to produce a valid AST for an empty file (previously, it was only handled in a special check after the parser completed). That allows the AST for an empty file to still retain information about comments and whitespace (so "empty file" really meaning no tokens/lexical elements, not necessarily no bytes).

[jhump]

This was the issue: adding a NoSourceNode to a FileNode was a poor choice. Also, it wasn't even necessary as the replacement code (to just delegate to NewFileNode) works and is much more clear.