Clean-up libfetchers headers

[fzakaria]

This is in the same vein as #14251

Motivation

I ran include-what-you-use over libflake as it seemed small to start.

steps:

> iwyu_tool.py -p build/ src/libfetchers -j 100 -- -Xiwyu --mapping_file=/nix/store/0zqzzg1mm1fman18svxkf5p4km4yq85p-include-what-you-use-0.24/share/include-what-you-use/boost-all.imp -Xiwyu --cxx17ns -Xiwyu --no_fwd_decls -Xiwyu --mapping_file=$(realpath iwyu.imp) > iwyu.out

> cd build

> fix_includes.py < ../iwyu.out

I then went through the header files and now I can more safely remove the header files clangd warns that are "not used".

Context

There is an ongoing debate on whether to do include-what-you-use (iwyu).

For:

• https://github.com/include-what-you-use/include-what-you-use/blob/master/docs/WhyIWYU.md

Against:

• https://danra.prose.sh/dont_use_iwyu

Note: You may still see "unused" header declaration warnings by clangd. From what I can understand, since I am running iwyu only on partial codebase, it is putting exactly the necessary includes for what is needed however includes from other parts of the codebase may still be giving the declarations for some headers that are included.
I saw this notably with includes for <mutex> for instance.

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[fzakaria]

This file now can handle the common mappings we want here as it grows.

[fzakaria]

This seemed like a small bug missing the pragma
I bet this could be an easy lint?

[fzakaria]

Interesting that sometimes my mapping file just converts these to git2.h but othertimes it replaces it with more.