Follow HTTP redirects (#328)

- Change HttpClient to follow all redirects except HTTPS to HTTP.
- Run language snippet tests with --no-cache and real PackageServer
  instead of pre-seeded cache.
  This increases HTTP test coverage and enables testing of package redirects.
- Change PackageServer to return 301 for request paths starting with /HTTP301/
  and 307 for request paths starting with /HTTP307/.
- Update some outdated test package checksums that apparently weren't verified.

pkl-core/src/test/files/LanguageSnippetTests/input/packages/packages1.pkl

@@ -16,7 +16,7 @@ examples {
     import("package://localhost:0/birds@0.5.0#/catalog/Ostritch.pkl")
   }
   ["importing while specifying checksum"] {
-    import("package://localhost:0/birds@0.5.0::sha256:3f19ab9fcee2f44f93a75a09e531db278c6d2cd25206836c8c2c4071cd7d3118#/catalog/Swallow.pkl")
+    import("package://localhost:0/birds@0.5.0::sha256:bfaf5281613d170a740505cc87561041f4e0cad1f0e6938bf94f7609f9a4673d#/catalog/Swallow.pkl")
   }
   ["reads"] {
     read("package://localhost:0/birds@0.5.0#/Bird.pkl")

pkl-core/src/test/files/LanguageSnippetTests/input/packages/packages2.pkl

@@ -20,6 +20,6 @@ examples {
     import("package://localhost:0/birds@0.5.0#/allFruit.pkl").fruitFiles
   }
   ["glob import while specifying checksum"] {
-    import*("package://localhost:0/birds@0.5.0::sha256:3f19ab9fcee2f44f93a75a09e531db278c6d2cd25206836c8c2c4071cd7d3118#/catalog/*.pkl")
+    import*("package://localhost:0/birds@0.5.0::sha256:bfaf5281613d170a740505cc87561041f4e0cad1f0e6938bf94f7609f9a4673d#/catalog/*.pkl")
   }
 }

pkl-core/src/test/files/LanguageSnippetTests/input/packages/redirects.pkl

@@ -0,0 +1,15 @@
+amends ".../snippetTest.pkl"
+
+examples {
+  ["permanent redirect is followed"] {
+    import("package://localhost:0/HTTP301/birds@0.5.0#/catalog/Swallow.pkl")
+  }
+
+  ["temporary redirect is followed"] {
+    import("package://localhost:0/HTTP307/birds@0.5.0#/catalog/Swallow.pkl")
+  }
+  
+  ["double redirect is followed"] {
+    import("package://localhost:0/HTTP301/HTTP307/birds@0.5.0#/catalog/Swallow.pkl")
+  }
+}

pkl-core/src/test/files/LanguageSnippetTests/output/packages/packages2.pcf

@@ -118,13 +118,13 @@ examples {
   }
   ["glob import while specifying checksum"] {
     new {
-      ["package://localhost:0/birds@0.5.0::sha256:3f19ab9fcee2f44f93a75a09e531db278c6d2cd25206836c8c2c4071cd7d3118#/catalog/Ostritch.pkl"] {
+      ["package://localhost:0/birds@0.5.0::sha256:bfaf5281613d170a740505cc87561041f4e0cad1f0e6938bf94f7609f9a4673d#/catalog/Ostritch.pkl"] {
         name = "Ostritch"
         favoriteFruit {
           name = "Orange"
         }
       }
-      ["package://localhost:0/birds@0.5.0::sha256:3f19ab9fcee2f44f93a75a09e531db278c6d2cd25206836c8c2c4071cd7d3118#/catalog/Swallow.pkl"] {
+      ["package://localhost:0/birds@0.5.0::sha256:bfaf5281613d170a740505cc87561041f4e0cad1f0e6938bf94f7609f9a4673d#/catalog/Swallow.pkl"] {
         name = "Swallow"
         favoriteFruit {
           name = "Apple"

pkl-core/src/test/files/LanguageSnippetTests/output/packages/redirects.pcf

@@ -0,0 +1,26 @@
+examples {
+  ["permanent redirect is followed"] {
+    new {
+      name = "Swallow"
+      favoriteFruit {
+        name = "Apple"
+      }
+    }
+  }
+  ["temporary redirect is followed"] {
+    new {
+      name = "Swallow"
+      favoriteFruit {
+        name = "Apple"
+      }
+    }
+  }
+  ["double redirect is followed"] {
+    new {
+      name = "Swallow"
+      favoriteFruit {
+        name = "Apple"
+      }
+    }
+  }
+}