Check bazel version: print a warning if version not in range

[smelc]

PR for issue #741

As discussed on slack, we don't to fail if the actual bazel version is not the expected one, we prefer to issue a warning. As a consequence, the code doesn't write anything if it cannot find the version (don't want to issue a warning about a possible warning..).

For a reason detailed here: bazelbuild/bazel#8305 checking bazel's version cannot be done anywhere, that is why it is done in haskell/repositories.bzl.

I did not find a preferred way how to issue a warning message, so I'm simply using Starlark's print function for our messages, please reconsider this.

Tests

In rules_haskell/tutorial, with bazel from APT

> sudo apt install bazel=1.2.1
> bazel --version
bazel 1.2.1
> bazel build //...
Starting local Bazel server and connecting to it...
DEBUG: /home/churlin/.cache/bazel/_bazel_churlin/b8c8fe86b6cf2c554ee68454e2b527a7/external/rules_haskell/haskell/private/versions.bzl:28:9: WARNING: bazel version is too old. Expected 2.0.0, but found: 1.2.1
... usual output

> sudo apt purge bazel; sudo apt install bazel=2.0.0
> bazel --version
bazel 2.0.0
> bazel build //...
Starting local Bazel server and connecting to it...
... usual output

> sudo apt purge bazel; sudo apt install bazel=2.1.0
> bazel --version
bazel 2.1.0
> bazel build //...
DEBUG: /home/churlin/.cache/bazel/_bazel_churlin/b8c8fe86b6cf2c554ee68454e2b527a7/external/rules_haskell/haskell/private/versions.bzl:31:9: WARNING: bazel version is too recent. Expected 2.0.0, but found: 2.1.0
... usual output

At the repo's root, with bazel from nix-shell

> bazel --version
bazel 2.0.0- (@non-git)
> bazel test //...
# tests pass
> bazel run //:buildifier
# nothing to format

Then with a modified nixkpgs/default.nix, to have bazel 2.1.0:

-  # 2020-01-29
-  sha256 = "1iy3iz91xx58k9wgswalipa8gxxdafqq82lisg6s16ml8y232f00";
-  rev = "42a195919a10ce70309c3666dfe8206e380fae1b";
+  # 2020-03-10
+  sha256 = "1kvyrs8g1zidrcs714k8g1gawrqln9x1nndpbfnblkglnzvvrac5";
+  rev = "3eea746d74d1fdfa57eef62b9159f5a51f23562a";

> bazel --version
bazel 2.1.0- (@non-git)
> bazel test //...
Starting local Bazel server and connecting to it...
DEBUG: /media/crypt1/repos/rules_haskell_741/haskell/private/versions.bzl:31:9: WARNING: bazel version is too recent. Expected 2.0.0, but found: 2.1.0- (@non-git)
# tests fail, because of an unexpected version of ghc (8.8.2 available, 8.6.5 expected)
> bazel run //:buildifier
DEBUG: /media/crypt1/repos/rules_haskell_741/haskell/private/versions.bzl:31:9: WARNING: bazel version is too recent. Expected 2.0.0, but found: 2.1.0- (@non-git)
# nothing to format

[aherrmann]

Thank you, looks good.