[path_provider] Remove win32

[stuartmorgan]

Per https://github.com/flutter/flutter/blob/master/docs/ecosystem/contributing/README.md#dependencies, we generally do not want third-party dependencies. path_provider in particular is a key part of the ecosystem (well over 1,000 packages depend directly on it, and transitively it's probably several times that at least), and thus the maintenance and security considerations are particularly acute.

This eliminates the dependency on win32, a large third-party dependency, in favor of direct FFI code written from scratch using the official Win32 reference documentation from Microsoft as the source. The only behavioral change that should result here is that the exceptions thrown in failure cases have changed, but they were never documented, and were entirely platform-specific, so it's relatively unlikely that people will be broken by that. (As noted in a TODO, the longer term solution is to provide real exceptions for this package, and use those across platforms.)

Fixes flutter/flutter#130940

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[loic-sharma]

package:win32's GUID.parse requires the {}s and -s. Should we do that as well?

[stuartmorgan]

I'm removing those symbols anyway to make the implementation simple, and since https://en.wikipedia.org/wiki/Universally_unique_identifier#Textual_representation says they are all valid I figured I would just accept all of them and relaxed the API accordingly. I didn't look at the win32 implementation so I don't know if there's a reason they are stricter; the only one I could think of would be efficiency, and I'm pretty confident that it's never going to matter in this use case. (If efficiency were ever actually important the way to go would be to make an API that's not string-based in the first place, and constants that built the GUIDs directly.)

[loic-sharma]

Sounds good to me, thanks for the thoughtful reply

[loic-sharma]

LGTM, nice work!

[auto-submit]

auto label is removed for flutter/packages/7073, due to - The status or check suite Mac_arm64 ios_platform_tests_shard_4 master has failed. Please fix the issues identified (or deflake) before re-applying this label.