Improve handling of precompiled C++ libraries

[hlopko]

Right now users can put precompiled libraries into cc_library.srcs attribute. This has some limitations, namely:

• It's not possible to link such library as whole-archive (well, in a sane way)
• It's not possible to group libraries into lib-groups
• There's no way of specifying if the library is static or dynamic, only the file extension is checked, which causes trouble on Windows with their .lib libraries.
• People use cc_library.srcs approach so cc_library can depend on cc_binary, but don't realize the consequences.

Solution might be a new rule, cc_import, that will provide support for all of above, with nice documentation explaining what can happen (diamonds).

Related: #70, #3323

[hlopko]

Feel free to comment on a design doc for this issue.