Add function for checking whether a repository URL has the right format

The existing code for doing this was exceptionally convoluted, so rather than exporting the components of that interface,
I added a simple function to serve as the exported interface for this functionality of the package.

Author: per1234

internal/libraries/repolist.go

@@ -74,6 +74,11 @@ func (repoMatcherIfDotGit) Match(url string) bool {
 	return strings.Index(url, "https://") == 0 && strings.LastIndex(url, ".git") == len(url)-len(".git")
 }
 
+// RepoURLValid returns whether the given URL has a valid format.
+func RepoURLValid(url string) bool {
+	return repoMatcherIfDotGit{}.Match(url)
+}
+
 // GitURLsError is the type for the unknown or unsupported repositories data.
 type GitURLsError struct {
 	Repos []*Repo

internal/libraries/repolist_test.go

@@ -24,11 +24,30 @@
 package libraries
 
 import (
+	"fmt"
 	"testing"
 
+	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
 
+func TestRepoURLValid(t *testing.T) {
+	testTables := []struct {
+		url       string
+		assertion assert.BoolAssertionFunc
+	}{
+		{"example.com", assert.False},
+		{"example.com/foo.git", assert.False},
+		{"http://example.com/foo.git", assert.False},
+		{"https://example.com/foo", assert.False},
+		{"https://example/com/foo.git", assert.True},
+	}
+
+	for _, testTable := range testTables {
+		testTable.assertion(t, RepoURLValid(testTable.url), fmt.Sprintf("URL: %s", testTable.url))
+	}
+}
+
 func TestRepoFolderPathDetermination(t *testing.T) {
 	repo := &Repo{URL: "https://github.com/arduino-libraries/Servo.git"}
 	f, err := repo.AsFolder()