Support package.json subpath imports

[dobesv]

Apply imports map from package.json to discovered dependencies. If the
dependency starts with '#' and we have a matching entry in "imports" in
the package.json we'll substitute the possible mapped imports in place of
that dependency.

Conditions can be well-known ones implemented by node, TypeScript, or webpack like
"import", "browser", "types", or "require". They can also be custom ones as configurable
in webpack configuration or using the enhanced-resolve package.

See also:

• https://nodejs.org/api/packages.html#subpath-imports
• https://www.typescriptlang.org/docs/handbook/esm-node.html
• https://webpack.js.org/configuration/resolve/#resolveconditionnames

Fixes #666

[dobesv]

I improved the code docs for that import map thing for the benefit of future maintainers trying to figure out what it's all about.

[rumpl]

LGTM, @dobesv if you don't mind, could you squash your commits before I merge?

[dobesv]

I'm AFK now, I'll have to do it tomorrow. Doesn't GitHub have a "squash and merge" option? Or does that require a paid plan?

[dobesv]

I squashed the commits

[rumpl]

Thanks!