Massimiliano Culpo 497fddfcb9 Fetching from URLs falls back to mirrors if they exist (#13881) ... Users can now list mirrors of the main url in packages. - [x] Instead of just a single `url` attribute, users can provide a list (`urls`) in the package, and these will be tried by in order by the fetch strategy. - [x] To handle one of the most common mirror cases, define a `GNUMirrorPackage` mixin to handle all the standard GNU mirrors. GNU packages can set `gnu_mirror_path` to define the path within a mirror, and the mixin handles setting up all the requisite GNU mirror URLs. - [x] update all GNU packages in `builtin` to use the `GNUMirrorPackage` mixin.		2019-12-20 14:32:18 -08:00
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spack	Fetching from URLs falls back to mirrors if they exist (#13881)	2019-12-20 14:32:18 -08:00