Prefixes of `Docfile.name` are inconsistent

[wtsnjp]

Docfile.name is a name of doc file only used for scoring. Their prefixes are inconsistent and probably depend on the sources of a docfile information (i.e., either from tlpdb or texdocs).

• Case 1: the name starts from texmf-dist

https://github.com/TeX-Live/texdoc/blob/6fec041ea8a141551c0e49f0da58a6d202d0cf79/spec/search/tlpdb_spec.rb#L27

• Case 2: the name starts from the dirname under the texmf-dist/doc tree

https://github.com/TeX-Live/texdoc/blob/6fec041ea8a141551c0e49f0da58a6d202d0cf79/spec/search/tlpdb_spec.rb#L8

The inconsistency is harmless for the function right now, but is slightly troublesome for testing and debugging.