fix: allow jsr packages to be mapped

[maraisr]

When wanting to map jsr packages to npm it would be nice to not crash.

example

await build({
  // ...
  mappings: {
    'jsr:@mr/object-identity': {
      name: 'object-identity',
      version: '^1'
    }
  } 
});

$ deno task build
error: Uncaught (in promise) "The following specifiers were indicated to be mapped to a package, but were not found:\n  * file:///Users/stuff/jsr:@mr/object-identity"

I am not 100% sure this will fix the issue, but sounds promising 🙏🏻

[CLAassistant]

All committers have signed the CLA.

[dsherret]

Thanks!