extensionMap is keyed by extension and valued by language, but the Julia row
was written backwards as `julia: "jl"`. checkCodeFile tests membership with
`extension in extensionMap`, so ".jl" never matched and a Julia artifact was
not a code file at all, while ".julia" resolved to a language named "jl".
Add both `jl: "julia"` and `julia: "julia"`, the same pair form the map already
uses for haskell/hs and elixir/ex, so no extension that classified before stops
classifying.
241a7b3b9b