Add support for mocking rpmmd

We want to test API methods which calls dnf. Unfortunately, calling dnf
is expensive operation - it requires network access and downloading
a lot of (meta)data. This commit changes the rpmmd implementation
so that it can be mocked.

internal/mocks/rpmmd/fixtures.go

@@ -0,0 +1,19 @@
+package rpmmd_mock
+
+import (
+	"github.com/osbuild/osbuild-composer/internal/rpmmd"
+)
+
+var BaseFixture = Fixture{
+	fetchPackageList{
+		rpmmd.PackageList{
+			{Name: "package1"},
+			{Name: "package2"},
+		},
+		nil,
+	},
+	depsolve{
+		nil,
+		nil,
+	},
+}

internal/mocks/rpmmd/rpmmd_mock.go

@@ -0,0 +1,33 @@
+package rpmmd_mock
+
+import "github.com/osbuild/osbuild-composer/internal/rpmmd"
+
+type fetchPackageList struct {
+	ret rpmmd.PackageList
+	err error
+}
+type depsolve struct {
+	ret []rpmmd.PackageSpec
+	err error
+}
+
+type Fixture struct {
+	fetchPackageList
+	depsolve
+}
+
+type rpmmdMock struct {
+	Fixture Fixture
+}
+
+func NewRPMMDMock(fixture Fixture) rpmmd.RPMMD {
+	return &rpmmdMock{Fixture: fixture}
+}
+
+func (r *rpmmdMock) FetchPackageList(repos []rpmmd.RepoConfig) (rpmmd.PackageList, error) {
+	return r.Fixture.fetchPackageList.ret, r.Fixture.fetchPackageList.err
+}
+
+func (r *rpmmdMock) Depsolve(specs []string, repos []rpmmd.RepoConfig) ([]rpmmd.PackageSpec, error) {
+	return r.Fixture.depsolve.ret, r.Fixture.depsolve.err
+}