The third_party svg directory has been a mix of pprof-specific code and
svgpan itself. This change moves the pprof-specific code out and adds a
license file. It also updates svgpan to 1.2.2 from
https://github.com/aleofreddi/svgpan while we are here. It also renames
the directory to svgpan to have all directories under third_party be
named precisely over the third party package they hold.

Also update the build instructions to use go get