Use cfg's for difference between librustdoc and crates.io version

[aDotInTheVoid]

Yeah, we should probably do that either way, to replace the increasing pile of regex we currently use.

Originally posted by @aDotInTheVoid in rust-lang/rfcs#3505 (comment)

[jalil-salame]

#42 reduces the regexes to just deleting the public export:

rustdoc-types/update.sh

Lines 11 to 13 in 44e2899

	curl -# https://raw.githubusercontent.com/${user}/${repo}/${branch}/src/rustdoc-json-types/lib.rs \
	| sed '/^pub type FxHashMap.*$/d' \
	> src/lib.rs

If this is still wanted I can make a PR to rust-lang/rust adding the cfg c:

[aDotInTheVoid]

I think #42 does enough to close this. I don't think it's worth having a reexport-hashmap feature.