config: detect duplicate & internal job names at parse time (#908)

Before this PR, config parsing would accept duplicate job names. `zrepl
daemon` would later fail to start with a panic. But tools like `zrepl
configcheck` would pass.

This PR adds a check to ensure job names are unique.

Similarly, internal job names were not being rejected by config parsing
Move that check to parse-time as well.

Last, drive-by change: remove `internal/config/config_include_test.go`
introduced in #856 . These aren't necessary because there is already a
wildcard test for all valid configs.
This PR adds the complimentary "invalid config" wildcard test.

internal/config/samples/invalid/duplicate_job_names/zrepl.yml

@@ -0,0 +1,24 @@
+jobs:
+  - type: snap
+    name: "my_job"
+    filesystems: {
+      "<": true,
+    }
+    snapshotting:
+      type: manual
+    pruning:
+      keep:
+        - type: last_n
+          count: 10
+
+  - type: snap
+    name: "my_job"
+    filesystems: {
+      "<": true,
+    }
+    snapshotting:
+      type: manual
+    pruning:
+      keep:
+        - type: last_n
+          count: 5

internal/config/samples/invalid/duplicate_job_names_with_include/included.yml

@@ -0,0 +1,12 @@
+jobs:
+  - type: snap
+    name: "my_job"
+    filesystems: {
+      "<": true,
+    }
+    snapshotting:
+      type: manual
+    pruning:
+      keep:
+        - type: last_n
+          count: 5

internal/config/samples/invalid/duplicate_job_names_with_include/zrepl.yml

@@ -0,0 +1,15 @@
+jobs:
+  - type: snap
+    name: "my_job"
+    filesystems: {
+      "<": true,
+    }
+    snapshotting:
+      type: manual
+    pruning:
+      keep:
+        - type: last_n
+          count: 10
+
+include:
+  - ./included.yml

internal/config/samples/invalid/internal_job_name/zrepl.yml

@@ -0,0 +1,12 @@
+jobs:
+  - type: snap
+    name: "_internal_job"
+    filesystems: {
+      "<": true,
+    }
+    snapshotting:
+      type: manual
+    pruning:
+      keep:
+        - type: last_n
+          count: 10