.packages: Move resource lookup & loading to a package and deprecate Resource class

[mit-mit]

#19430 will introduce support for getting the package root. Based on that we should move the implementation of what is in the Resource class into a package, and discontinue the Resource class.

[sethladd]

Related: #24499

[sethladd]

resource package placeholder created, @lrhn is an uploader: https://pub.dartlang.org/packages/resource

[mit-mit]

CR: https://codereview.chromium.org/1374953002/

[mit-mit]

And the CR for the package: https://codereview.chromium.org/1387163002/

[kevmoo]

Resource class is deprecated in 1.13 – keeping this open for 1.14 to track work on the package

[mit-mit]

@kevmoo, we should land this now, right?