feat: make deno vendor work with an existing import map by modifying it

[dsherret]

This is based on an idea from @rtpg:

1. Moves the import_map.json to be in the root directory instead of vendor folder.
2. Specific usages of remote dependencies in local code is added to the "imports" (ex. "https://localhost/mod.ts": "./vendor/localhost/mod.ts") instead of a general one (ex. "https://localhost/": "./vendor/localhost/")... so when a new remote specifier is used then deno vendor needs to be run again.
3. Strips all vendor output folder entries from the import map before vendoring in order to allow re-vendoring.

Closes #13697

[dsherret]

This sadly does not work because we need to modify import map entries like "$twind": "https://esm.sh/[email protected]" to point to the vendored location. We're going to continue using a vendor/import_map.json file. I will open a new PR that makes these changes.