Document configuration-specific imports

[mit-mit]

What information is missing?

We seem to be missing documentation on configuration specific imports. They are mentioned briefly here in the creating package docs: https://dart.dev/guides/libraries/create-library-packages#conditionally-importing-and-exporting-library-files

How would you like us to fix this problem?

Should be mentioned alongside the docs for imports, probably around here: https://dart.dev/guides/packages#importing-libraries-from-packages

Once we do that, we should link to it from the creating package docs.

[mit-mit]

cc @atsansone

@lrhn can probably help with eng. input

[mit-mit]

Here's the original proposal: https://github.com/lrhn/dep-configured-imports/blob/master/DEP-configured-imports.md

[mit-mit]

I'd like to adjust this to P2. There is a lot of mystery on how these work/