Reference implementation of scopes

[guybedford]

This provides a simple implementation of scoped resolution with the various fallback tests.

The gist of the approach is:

• Sort scopes, and scope imports just like imports
• Generalize the matcher into a resolveImportsMatch function which is used on scopes
• The first best matching scope is checked for import resolution, falling back to the global imports.

Because of the way scopes are normalized during parsing, we end up needing to check for parentURL[normalizedScopeURL.length] === '/' || parntURL[normalizedScopeURL.length - 1] === '/' because scopes may or may not end in '/'. One optimization here might be to normalize scopes to always be with or without the trailing slash (possibly without?), but that seemed to affect quite a few of the parsing tests when I tried it. I could certainly implement that as well though.

Any other thoughts re edge cases very welcome, but I think most are already covered by the base import maps themselves.

[domenic]

I'm going to add to this PR to implement no-trailing-slash = exact matching, trailing-slash = prefix matching. For now that seems best to me. But let's keep #73 open and discuss more fully there; we can change after landing this.