Rename package to PkgGraphs.jl

[tfiers]

• More in line w/ julia pkg naming convention (plurals)
• Sounds better

Better do soonish, takes 3 days for PR merge of sth new right.
Also, it'll complain that similar to existing package (ourself).
sooo..

ah, here we go: https://github.com/JuliaRegistries/General#how-do-i-rename-an-existing-registered-package

ok so i indeed do that:

• rename gh repo, filenames (PkgGraph.jl), and search-and-replace PkgGraph errywhere.
• generate a new UUID
• make pr to General to change url in old pkg, as in https://github.com/JuliaRegistries/General/pull/40190/files
  • → Update PkgGraph URL (renamed repo/package) JuliaRegistries/General#74746
• (note this in the changelog)
• register as new package (incr v → registerbot), and "Comment on the PR, that this is a rename."
  (with "[noblock]" in the comment) . Wait 3 days ("even if manually merged")
  • prep v0.2 release
    • backfill changelog (using v0.1...d9f4153 (commit where I introduced cl))
    • rollover changelog
    • update install instructions (v0.1 not relevant anymore)
  • → version bump commit: 903e8df
  • For TagBot: that only happens when PR merged (see issue for v0.1) (✔)