I'm not sure why an extra "dnscrypt-proxy" was added to the module name when the rest of the URL was updated, but it resulted in the following:
```
$ go test -mod=vendor ./...
ok github.com/DNSCrypt/dnscrypt-proxy/dnscrypt-proxy/dnscrypt-proxy 0.173s
ok github.com/DNSCrypt/dnscrypt-proxy/dnscrypt-proxy/test 0.006s
$ go list ./...
github.com/DNSCrypt/dnscrypt-proxy/dnscrypt-proxy/dnscrypt-proxy
github.com/DNSCrypt/dnscrypt-proxy/dnscrypt-proxy/test
```
Not critical, but it looks wrong that these packages will not be found at those URLs.
To simulate failures opening a cache file, fixtures are written without the read permission bits.
Since Unix permission bits have no meaning on Windows, a slightly more complicated solution is required to achieve the same permissions.
Thankfully, there's a library to abstract that already.
Tests cover most of the cache and download related code paths and specify the expected result of various starting states and external failure modes.
Where the current code's behaviour doesn't match a test's expectations, the test is disabled and annotated with a TODO until it can be fixed.
Added dependency on `github.com/powerman/check` and ran `go mod vendor`.
```console
$ go mod tidy -v
(snip)
unused github.com/agl/ed25519
```
Also add base .gitattributes file to normalize line endings in the repository across differing developer environments.
