github: fixes and improvements

- "file" -> "asset"
- "version" -> "tag" or "release" (where appropriate)
- fix updater.go for upstream changes
- make printed log messages more similar to those of other modules
- move http request function(s) to separate file "request.go"
- remove the concept of a "Mod"; we're using "Repo"s (GitHub repositories) instead
- remove unnecessary fields in structs
- use sha256 instead of sha1 for asset checksums

Signed-off-by: unilock <unilock@fennet.rentals>

github/request.go

@@ -0,0 +1,33 @@
+package github
+
+import (
+	"fmt"
+	"net/http"
+)
+
+const ghApiServer = "api.github.com"
+
+type ghApiClient struct {
+	httpClient *http.Client
+}
+
+var ghDefaultClient = ghApiClient{&http.Client{}}
+
+func (c *ghApiClient) makeGet(slug string) (*http.Response, error) {
+	req, err := http.NewRequest("GET", "https://" + ghApiServer + "/repos/" + slug + "/releases", nil)
+	if err != nil {
+		return nil, err
+	}
+
+	req.Header.Set("Accept", "application/vnd.github+json")
+
+	resp, err := c.httpClient.Do(req)
+	if err != nil {
+		return nil, err
+	}
+
+	if resp.StatusCode != 200 {
+		return nil, fmt.Errorf("invalid response status: %v", resp.Status)
+	}
+	return resp, nil
+}