Make a value of 'imports' property of py_library rule visible in Skylark

[nuh-temp]

In most cases that parameter indicates a path to library's sources and widely used along with new_[git|local]_repository rules in order to setup import path for external python libraries.
In some cases all such libraries should go through additional processing, like for example, if library is used with AppEngine you need to put it into a folder which is known to AppEngine to be able to import it properly.

[hermione521]

@lberki what do you think?

[evanj]

I just ran into this while attempting to figure out how to build a Python mono-repo that puts separate Python packages in different sub-directories. We would like to be able to use PEX, since that can package pre-built wheels which Bazel doesn't support yet. While trying to figure out how to write custom rules to support this, I discovered that I can't because I can't access imports from Skylark. See benley/bazel_rules_pex#36 for more motivation.

[duggelz]

I work around this by parsing the imports list from the generated stub files, but it's a brittle solution that doesn't work on Windows, and occasionally breaks for other reasons.

[c4urself]

ran into this as well -- there are workarounds but the cleanest way is to have imports available.