Finding the URL of methods doesn't work for packages added (not dev'ed)

[oxinabox]

The code that finds the URL for the source for a method doesn't work for code that was installed via Pkg.add.
Since as of Pkg3 that no longer means it is a git repo.

julia/base/methodshow.jl

Lines 200 to 238 in f4c7779

	function url(m::Method)
	M = m.module
	(m.file == :null || m.file == :string) && return ""
	file = string(m.file)
	line = m.line
	line <= 0 || occursin(r"In\[[0-9]+\]", file) && return ""
	Sys.iswindows() && (file = replace(file, '\\' => '/'))
	libgit2_id = PkgId(UUID((0x76f85450_5226_5b5a,0x8eaa_529ad045b433)), "LibGit2")
	if inbase(M)
	if isempty(Base.GIT_VERSION_INFO.commit)
	# this url will only work if we're on a tagged release
	return "https://github.com/JuliaLang/julia/tree/v$VERSION/base/$file#L$line"
	else
	return "https://github.com/JuliaLang/julia/tree/$(Base.GIT_VERSION_INFO.commit)/base/$file#L$line"
	end
	elseif root_module_exists(libgit2_id)
	LibGit2 = root_module(libgit2_id)
	try
	d = dirname(file)
	return LibGit2.with(LibGit2.GitRepoExt(d)) do repo
	LibGit2.with(LibGit2.GitConfig(repo)) do cfg
	u = LibGit2.get(cfg, "remote.origin.url", "")
	u = match(LibGit2.GITHUB_REGEX,u).captures[1]
	commit = string(LibGit2.head_oid(repo))
	root = LibGit2.path(repo)
	if startswith(file, root) || startswith(realpath(file), root)
	"https://github.com/$u/tree/$commit/"*file[length(root)+1:end]*"#L$line"
	else
	fileurl(file)
	end
	end
	end
	catch
	return fileurl(file)
	end
	else
	return fileurl(file)
	end
	end

I think I have code to fix this.
It involves trying to load Pkg (just like current code already tries to load LibGit2).
Then asking Pkg to check the registry for the git url is,
as well as checking what the current version (and thus tag) is.

[oxinabox]

CF: JuliaLang/Pkg.jl#960