Implement //ci:release-validate-deps (#95)

The `//ci:release-validate-deps` can be used from the CI job to ensure that we're depending on a release instead of a snapshot dependency.

For example, if we are to release `graknlabs_grakn_core` which depends on a snapshot version of `graknlabs_common` will fail with the following error message:

```
bazel run @graknlabs_build_tools//ci:release-validate-deps graknlabs_graql graknlabs_common

RuntimeError: This commit is not releasable because there are one or more snapshot 
dependencies: ['graknlabs_common']. Check that every dependencies listed in 
dependencies/graknlabs/dependencies.bzl are all release dependencies 
(ie., depends on a tag instead of a commit).
```

If every dependency is a release dependency, it will succeed:
```
bazel run @graknlabs_build_tools//ci:release-validate-deps graknlabs_graql graknlabs_common

This repository is releasable because the dependencies
are all release dependencies: ['graknlabs_common', 'graknlabs_graql']
```

ci/BUILD

@@ -61,6 +61,12 @@ py_binary(
     ]
 )
 
+py_binary(
+    name = "release-validate-deps",
+    srcs = ["release-validate-deps.py"],
+    main = "release-validate-deps.py",
+)
+
 sh_binary(
     name = "install-bazel-rbe",
     srcs = ["install-bazel-rbe.sh"]

ci/release-validate-deps.py

@@ -0,0 +1,27 @@
+import os
+import sys
+
+if __name__ == '__main__':
+    deps_path = os.path.join('dependencies', 'graknlabs', 'dependencies.bzl')
+    snapshot_dependencies = []
+    with open(os.path.join(os.getenv("BUILD_WORKSPACE_DIRECTORY"), deps_path)) as deps_file_object:
+        deps_content = deps_file_object.read().splitlines()
+        for arg in sys.argv[1:]:
+            dependency = list(filter(lambda line: line.endswith('@' + arg), deps_content))
+            if len(dependency) == 1:
+                depend_by_tag = dependency[0].strip().startswith('tag')
+                if not depend_by_tag:
+                    snapshot_dependencies.append(arg)
+            else:
+                raise RuntimeError('Invalid sync-marker found for {}. '
+                                   'There is supposed to be exactly one sync-marker '
+                                   'but {} occurrences found.'.format(arg, len(dependency)))
+    if len(snapshot_dependencies) == 0:
+        print('This commit is releasable because the dependencies are '
+              'all released dependencies: {}'.format(sys.argv[1:]))
+    else:
+        raise RuntimeError('This commit is not releasable because '
+                           'there are one or more snapshot dependencies: {}. '
+                           'Check that every dependencies listed in {} are all released dependencies '
+                           '(ie., depends on a tag instead of a commit).'
+                           .format(snapshot_dependencies, deps_path))