add import and directory support to export map subpath exports

[thescientist13]

Related Issue

resolves #671

Summary of Changes

1. If subpath export is a directory, add each file to the importMap
2. Added support for subpath exports that are of type import

Blocking #611 and AnalogStudiosRI/www.analogstudios.net#8

Discussion

I'm thinking now, based on the nature of this fix, if we actually stillvneed walkModule to manually crawl all the imports and exports of every dependency to add to the importMap?. I would be curious to try now with this bug fix to see if our importMap still works without it? Maybe should happen as part of a minor release though, so this PR can just go out as a patch.

Additionally, not sure if a sub path export is a directory, does that mean we should recursively export everything? Could be part of an issue that also addresses the above question re: walkModule, which coincidentally, I've playing around with in #668

Update: made a quick test ( #672 (comment) ) so seems like it is still needed.

[thescientist13]

As a simple test, tried commenting out the walkModule function calls and at least for this version of Lit, got this error, so seems like it is still needed.

Can re-evaluate further in #674 if desired, since no big changes you should really happen in a patch fix, IMO.