Implementation of maven_jar rule in Skylark.

**Experimental**

This is an initial implementation of the maven_jar rule in Skylark, targeted at
the FRs in issue #1410.

Implemented a wrapper around the maven binary to pull dependencies from
remote repositories into a directory under {output_base}/external.

Attributes `name`, `artifact`, `repository`, `sha1` have been implemented,
but not `server`.

Caveat: this rule assumes that the Maven dependency is installed in the
system. Hence, the maven_skylark_test integration tests are tagged with
"manual" and commented out because the Bazel CI isn't configured with
the Maven binary yet.

Added a serve_not_found helper for 404 response tests.

Usage:

```
load("@bazel_tools//tools/build_defs/repo:maven_rules.bzl", "maven_jar")

maven_jar(
    name = "com_google_guava_guava",
    artifact = "com.google.guava:guava:18.0",
    sha1 = "cce0823396aa693798f8882e64213b1772032b09",
    repository = "http://uk.maven.org/maven2",
)
```

With regards to server, there are some limitations with retrieving a
maven_server's attribute at Loading Phase without the use of hacky macros
(issue #1704), and even if macros are used, the maven_server is not treated as
an actual dependency by maven_jar. There is a test (`test_unimplemented_server_attr`)
to ensure that the error message to shown to users if they use the server
attribute with this rule.

--
Change-Id: I167f9d13835c30be971928b4cc60167a8e396893
Reviewed-on: https://bazel-review.googlesource.com/c/5770
MOS_MIGRATED_REVID=133971809

1 files changed, 5 insertions, 0 deletions

diff --git a/WORKSPACE b/WORKSPACE
index ec8dff751f..b06d57f180 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -52,3 +52,8 @@ bind(name = "xcrunwrapper", actual = "@bazel_tools//tools/objc:xcrunwrapper")
 
 bind(name = "protobuf/java_runtime", actual = "//third_party/protobuf:protobuf")
 bind(name = "protobuf/javalite_runtime", actual = "//third_party/protobuf:protobuf-lite")
+
+# For Skylark tests at //src/test/shell/bazel:maven_skylark_test
+# Uncomment the following lines, and the test in src/test/shell/bazel/BUILD to run it.
+# load("//tools/build_defs/repo:maven_rules.bzl", "maven_dependency_plugin")
+# maven_dependency_plugin()