Fix port data source import on Windows

[j-maas]

When trying to use a port-data-source.js on Windows, I run into another path-related issue:

Trace: DataSource.Port.send "imageSources" was called, but I couldn't find a port definitions file. Create a 'port-data-source.ts' or 'port-data-source.js' file and export a imageSources function.
    at C:\Users\Johannes\Code\elm-pages-3-alpha-starter\elm-pages\generator\src\request-cache.js:84:19

The cause is that we import() the file using an absolute path, but Windows-style absolute paths do not work and we have to use file:// URLs instead.

[j-maas]

This is trickier than I thought. If I provoke a syntax error, the file will not be compiled. But the dev server will actually start. So when I later call a data source port, I get an error saying it couldn't find the file which is confusing: It means it can't find the compiled file, since compilation failed, but the data-source-port.js actually exists.

I've tried a few things, but I'm getting tired and will leave this as is for now. I think it might already be an improvement and we could weed out this case separately. But you decide what's the best option for you. ;)

[dillonkearns]

Sounds good, thanks so much for the PR! That's especially helpful for windows fixes since I don't have an environment to reproduce and iterate on fixes, so I really appreciate it!

The distinction between syntax errors vs. missing port-data-source file I think is something I'll have to deal with regardless of platform, so I'll go ahead and merge this in and work on that separately (that was already on my todo list). Thank you!