Support @__FILE__ in Pages in @content and @index blocks

[goerz]

JuliaDocs/DocumenterCitations.jl#22 (comment) proposed accepting @__FILE__ in the Pages attribute of a @bibliography block to indicate the current page. I've implemented this in JuliaDocs/DocumenterCitations.jl#48.

For consistency (and because this seems like a useful feature, making it easier to move around/rename .md files in the documentation), it would be nice if Documenter itself would also support this in the Pages attribute of @content and @index blocks. Probably also in @autodocs, although Pages in @autodocs is handled differently from @content and @index.

The function I'm using to handle @__FILE__ in DocumenterCitations is this:

# Deal with `@__FILE__` in `Pages`, convert it to the name of the current file.
 function _resolve__FILE__(Pages, page)
     __FILE__ = let ex = Meta.parse("_ = @__FILE__", 1; raise=false)[1]
         # What does a `@__FILE__` in the Pages list evaluate to?
         # Cf. `Core.eval` in `parse_bibliography_block`.
         # Should be the string "none", but that's an implementation detail.
         Core.eval(Main, ex.args[2])
     end
     result = String[]
     for name in Pages
         if name == __FILE__
             # Replace @__FILE__ in Pages with the current file:
             name = basename(page.source)
             @debug "__@FILE__ -> $(repr(name)) in Pages attribute of @bibliography block on page $(page.source)"
         end
         push!(result, name)
     end
     return result
 end

It's applied to the parsed/eval'ed Pages list.

Issue #760 is related, but seems mostly about @example and @eval (which probably need pre-processing instead of post-processing)

[mortenpi]

This seems like a good idea to me.