better URLs for package text/html method display

[stevengj]

For Base methods, something like @which sum([1,2,3]) gives text/html output (e.g. in IJulia) with a useful github link, for example:

links https://github.com/JuliaLang/julia/tree/80516ca20297a67b996caa08c38786332379b6a5/base/reducedim.jl#L648

However, that doesn't work for packages, and now that so many functions have been moved to the stdlib, we get things like:

which uselessly links to a file:///Users/osx/buildbot/slave/package_osx64/build/usr/share/julia/stdlib/v1.1/LinearAlgebra/src/generic.jl on the buildbot machine.

For the stdlib at least, and ideally also for packages where we have a github URL and commit, it would be nice to link to a useful URL.

[fredrikekre]

Duplicate of #26314?

[stevengj]

@fredrikekre, not a duplicate, because @edit and @less need to find a local file, whereas the URL should ideally point to github like it does for Base methods.