Enable spaces in exports_files

[abergmeier]

We put a binary Python archive in an external repo.

For that repository we had the BUILD file:

all_files = glob(["**"])

exports_files(srcs=all_files)

which is making Bazel complain about a target having a space (setuptools has a file with spaces in basename). Which I understand as a limitation for targets, but it is not always an option to remove paths with spaces from thirdparty libraries.

[philwo]

The fact that not all filenames are valid label identifiers is unfortunately a long-standing issue with Bazel. I totally agree with you here, that this should just work. Unfortunately a lot of code currently relies on the fact that labels contain no special characters or whitespace - it will take a while to sort this all out. We hope to fix this soon, though!

[kchodorow]

Possibly a dup of #167.

[damienmg]

Definitely dup of #167