This commit adds automatic expansion of "imports" field in "deno.json"
file.
If "npm:" or "jsr:" imports are encountered we automatically try to add
a "directory" remapping.
Previously users had to specify entries for both `foo` and `foo/` to be
able to import like
`import { symbol1 } from "foo";` and `import { symbol2 } from
"foo/some_file.js"`:
```
{
"imports": {
"foo": "npm:@foo/bar",
"foo/": "npm:/@foo/bar/",
}
```
With this change users can only add entry for `foo`:
```
{
"imports": {
"foo": "npm:@foo/bar",
}
```
The entry for `foo/` will be provided automatically.
Similarly if user provides "directory" remapping explicitly, we will not
overwrite it.
