fix: import maps does not work on local files

[aiotter]

When you specify a local file as an import map, Lume raises an error even when the specified file is a valid import map.

$ lume --serve -- --import-map=./import_map.json
Error: Unable to load the import map file
- mapFile: ./import_map.json
    at getImportMap (https://deno.land/x/lume@v1.4.1/ci.ts:53:13)
    at async getArgs (https://deno.land/x/lume@v1.4.1/ci.ts:95:33)
    at async main (https://deno.land/x/lume@v1.4.1/ci.ts:103:32)
    at async (https://deno.land/x/lume@v1.4.1/cli.ts:150:5)

Caused by TypeError: Invalid URL
    at Object.opSync (deno:core/01_core.js:149:12)
    at opUrlParse (deno:ext/url/00_url.js:47:27)
    at new URL (deno:ext/url/00_url.js:320:20)
    at new Request (deno:ext/fetch/23_request.js:241:27)
    at (deno:ext/fetch/26_fetch.js:396:29)
    at fetch (deno:ext/fetch/26_fetch.js:392:12)
    at getImportMap (https://deno.land/x/lume@v1.4.1/ci.ts:48:33)
    at getArgs (https://deno.land/x/lume@v1.4.1/ci.ts:95:39)
    at main (https://deno.land/x/lume@v1.4.1/ci.ts:103:38)

This PR resolves this.

[oscarotero]

Nice!
Just one comment: maybe it's better to use toFileUrl function of the std library: https://deno.land/std@0.120.0/path#tofileurl. It looks more stronger and it can handle paths on Windows environment.

[aiotter]

Wow, that's handy. I'll update the code tomorrow.

[aiotter]

Updated!
I have no windows environment so I couldn't test it on windows. On macOS it works properly.

[oscarotero]

Perfect. Thank you!