feat: Add flit-core support

Signed-off-by: Pascal Zimmermann <pascal.zimmermann@theiotstudio.com>

# Conflicts:
#	manifest.yml

# Conflicts:
#	go.mod
#	go.sum

Author: ZPascal

README.md

@@ -24,7 +24,9 @@ To build this buildpack, run the following commands from the buildpack's directo
 1. Install buildpack-packager
 
     ```bash
-    go install github.com/cloudfoundry/libbuildpack/packager/buildpack-packager
+    go get github.com/cloudfoundry/libbuildpack
+    cd ~/go/src/github.com/cloudfoundry/libbuildpack && GO111MODULE=on go mod download
+    cd packager/buildpack-packager &&  GO111MODULE=on go install
     ```
 
 1. Build the buildpack

src/python/supply/supply.go

@@ -743,7 +743,7 @@ func (s *Supplier) RunPipVendored() error {
 // dependencies - wheel and setuptools. These are packaged by the dependency
 // pipeline within the "pip" dependency.
 func (s *Supplier) InstallCommonBuildDependencies() error {
-	var commonDeps = []string{"wheel", "setuptools"}
+	var commonDeps = []string{"wheel", "setuptools", "flit-core"}
 	tempPath := filepath.Join("/tmp", "common_build_deps")
 	if err := s.Installer.InstallOnlyVersion("pip", tempPath); err != nil {
 		return err

src/python/supply/supply_test.go

@@ -633,10 +633,11 @@ MarkupSafe==2.0.1
 
 	Describe("InstallCommonBuildDependencies", func() {
 		Context("successful installation", func() {
-			It("runs command to install wheel and setuptools", func() {
+			It("runs command to install wheel, setuptools and flit-core", func() {
 				mockInstaller.EXPECT().InstallOnlyVersion("pip", "/tmp/common_build_deps")
 				mockCommand.EXPECT().Execute(buildDir, gomock.Any(), gomock.Any(), "python", "-m", "pip", "install", "wheel", "--no-index", "--upgrade-strategy=only-if-needed", "--find-links=/tmp/common_build_deps")
 				mockCommand.EXPECT().Execute(buildDir, gomock.Any(), gomock.Any(), "python", "-m", "pip", "install", "setuptools", "--no-index", "--upgrade-strategy=only-if-needed", "--find-links=/tmp/common_build_deps")
+				mockCommand.EXPECT().Execute(buildDir, gomock.Any(), gomock.Any(), "python", "-m", "pip", "install", "flit-core", "--no-index", "--upgrade-strategy=only-if-needed", "--find-links=/tmp/common_build_deps")
 
 				Expect(supplier.InstallCommonBuildDependencies()).To(Succeed())
 			})