Add support for wild-card stacks in dependencies

Signed-off-by: Sambhav Kothari <skothari44@bloomberg.net>

postal/buildpack.go

@@ -73,7 +73,7 @@ func parseBuildpack(path, name string) ([]Dependency, string, error) {
 
 func stacksInclude(stacks []string, stack string) bool {
 	for _, s := range stacks {
-		if s == stack {
+		if s == stack || s == "*" {
 			return true
 		}
 	}

postal/service_test.go

@@ -86,6 +86,14 @@ stacks = ["some-stack"]
 uri = "some-uri"
 version = "4.5.6"
 strip-components = 1
+
+[[metadata.dependencies]]
+id = "some-other-entry"
+sha256 = "some-sha"
+stacks = ["*"]
+uri = "some-uri"
+version = "4.5.6"
+strip-components = 1
 `)
 		Expect(err).NotTo(HaveOccurred())
 
@@ -115,6 +123,21 @@ strip-components = 1
 			}))
 		})
 
+		context("when the dependency has a wildcard stack", func() {
+			it("is compatible with all stack ids", func() {
+				dependency, err := service.Resolve(path, "some-other-entry", "", "random-stack")
+				Expect(err).NotTo(HaveOccurred())
+				Expect(dependency).To(Equal(postal.Dependency{
+					ID:              "some-other-entry",
+					Stacks:          []string{"*"},
+					URI:             "some-uri",
+					SHA256:          "some-sha",
+					Version:         "4.5.6",
+					StripComponents: 1,
+				}))
+			})
+		})
+
 		context("when there is NOT a default version", func() {
 			context("when the entry version is empty", func() {
 				it("picks the dependency with the highest semantic version number", func() {