Convert to go modules and simplify package structure

[jschaf]

There's two commits here. The first uses go modules instead of GOPATH. The second commit simplifies the package structure to use pkg for library code and cmd for binaries.

Working

• make esbuild succeeds and rebuilds for changes in pkg/* and cmd/*.
• make test succeeds.
• make update-version-go succeeds and updates the verison.
• make platform-all succeeds.
• make clean succeeds.
• make demo-three succeeds.
• make bench-three succeeds.

Not working

• make test262 hangs.

[jschaf]

Oh, I missed your intent from the other PRs (#9) I'll close.