fix: match only against the end of a file path.

[pat]

Otherwise if there happens to be .rb in a directory name, that'll be incorrectly modified as well.

I haven't added a test as this code is part of loading everything, rather than something that occurs once the library is loaded.

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[broksonic21]

thanks for this one - we ran into it too (see #655 )

[jcavena]

Thanks for the fix!