Fixed loading of Library given a non-absolute path (#2265)

* Fixed loading of Library given a non-absolute path

Givin a relative path is not ideal, but at least allows the CLI to
compile the sketch correctly.

* Added test

arduino/libraries/libraries_test.go

@@ -54,12 +54,16 @@ func TestLibLayoutAndLocationJSONUnMarshaler(t *testing.T) {
 
 func TestLibrariesLoader(t *testing.T) {
 	{
-		lib, err := Load(paths.New("testdata", "TestLib"), User)
+		libPath := paths.New("testdata", "TestLib")
+		lib, err := Load(libPath, User)
 		require.NoError(t, err)
 		require.Equal(t, "TestLib", lib.Name)
 		require.Equal(t, "1.0.3", lib.Version.String())
 		require.False(t, lib.IsLegacy)
 		require.False(t, lib.InDevelopment)
+		absPath, err := libPath.Abs()
+		require.NoError(t, err)
+		require.Equal(t, lib.InstallDir.String(), absPath.String())
 	}
 	{
 		lib, err := Load(paths.New("testdata", "TestLibInDev"), User)

arduino/libraries/loader.go

@@ -28,6 +28,13 @@ import (
 
 // Load loads a library from the given LibraryLocation
 func Load(libDir *paths.Path, location LibraryLocation) (*Library, error) {
+	if !libDir.IsAbs() {
+		if abs, err := libDir.Abs(); err == nil {
+			libDir = abs
+		} else {
+			return nil, err
+		}
+	}
 	if libDir.Join("library.properties").Exist() {
 		return makeNewLibrary(libDir, location)
 	}