Add mirror support for `vcpkg_download_distfile` about vcpkg HOT 5 CLOSED

Do you think about a local cache ? Like downloading all libraries and ship them along with vcpkg ?
What about mirror downloads ?

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I think mirrors are definitely a great idea.

Additionally, we already first check the downloads\ folder before making external contact so you can (ahead of time) download and store all the source code you depend on, then copy them to downloads\ before running vcpkg install curl.

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I'm working on this issue right now.
If the file is successfully downloaded but I get the wrong hash, should I try a mirror or print a FATAL_ERROR ?

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In #83 I opted for the second option: once a file successed to download, I never try to download anymore, even if the HASH doesn't match.

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When I pre-download files, sometimes I have to add a prefix like 'msys-'. If you look in the downloads temp folder, it will create an empty file while attempting to download and this will help give a clue of the filename it expects, which is not always the same as the filename it is downloading.

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