Support file:// scheme for repo URLs

In a repo.yaml file, instead of just the http:// or https:// scheme,
the user might want to specify a local path for an repo, e.g.

    repositories:
    - arch: x86_64
      baseurl: file:///RPMS
      name: local

And then use that with fetch, e.g.

    % bazeldnf fetch
    INFO[0000] Resolving repomd.xml from file:///RPMS/repodata/repomd.xml
    INFO[0000] Loading primary file from file:///RPMS/repodata/<HASH>-primary.xml.gz

To address this, update the Getter implementation to parse the URL
using net/url Parse, and the call a handler that opens a local file
and returns a wrapper http.Response when the scheme is "file".

pkg/repo/fetch.go

@@ -8,6 +8,7 @@ import (
 	"io"
 	"net/http"
 	"net/url"
+	"os"
 	"path"
 	"path/filepath"
 	"strings"
@@ -186,7 +187,7 @@ func (r *RepoFetcherImpl) fetchFile(fileType string, repo *bazeldnf.Repository,
 	log.Infof("Loading %s file from %s", fileType, fileURL)
 	resp, err := r.Getter.Get(fileURL)
 	if err != nil {
-		return fmt.Errorf("Failed to load promary repository file from %s: %v", fileURL, err)
+		return fmt.Errorf("Failed to load primary repository file from %s: %v", fileURL, err)
 	}
 	sha := sha256.New()
 	defer resp.Body.Close()
@@ -214,8 +215,29 @@ type Getter interface {
 
 type getterImpl struct{}
 
-func (*getterImpl) Get(url string) (resp *http.Response, err error) {
-	return http.Get(url)
+func fileGet(filename string) (*http.Response, error) {
+	fp, err := os.Open(filename)
+	if err != nil {
+		return nil, err // skipped wrapping the error since the error already begins with "open: "
+	}
+
+	resp := &http.Response{
+		Status:     "OK",
+		StatusCode: http.StatusOK,
+		Body:       fp,
+	}
+	return resp, nil
+}
+
+func (*getterImpl) Get(rawURL string) (*http.Response, error) {
+	u, err := url.Parse(rawURL)
+	if err != nil {
+		return nil, fmt.Errorf("Failed to parse URL: %w", err)
+	}
+	if u.Scheme == "file" {
+		return fileGet(u.Path)
+	}
+	return http.Get(rawURL)
 }
 
 func toHex(hasher hash.Hash) string {