Vendor all dependencies into our version control

[cscheid]

This introduces src/vendor in addition to src/resources/vendor and a new "merged" import map that we can use during development. Most importantly, this should make it so that CI doesn't need to touch skypack. Every time src/import_map.json or src/resources/vendor/import_map.json changes, our configure scripts should revendor everything necessary, as well as recreate the merged import map.

There are a few things to decide on:

• What's the best name for the new import map?
• Should we rm the old vendored dependencies or mv them to a temporary spot like I did? I'd prefer not risk a shell script bug rm -rfing more than necessary.

[cscheid]

(It's really hilarious that this failed because somehow we're still going to the web and so the 500 error appeared here. It's gotten so bad that it might as well be deterministic)