Export pdk/build_defs.bzl so that StandardCellInfo can be used (and tested) by XLS

[tcal-x]

Not sure if I need to export open_road_configuration as well, or just the entry point? The goal is to access do_not_use_cell_list. This is the only export I need internally.

[QuantamHD]

Bzl files don't require an export to use them in other bzl files

[tcal-x]

Bzl files don't require an export to use them in other bzl files

There is a meta-Bazel test //xls/build_rules:xls_rules_bzl_parse_file_test that does need this export.

This is the failure from XLS bazel testing (line breaks added for readability):

ERROR: /home/tcal/xls-export/xls/build_rules/BUILD:126:12: 
no such target '@rules_hdl//pdk:build_defs.bzl': 
target 'build_defs.bzl' not declared in package 'pdk' defined by
 /usr/local/google/home/tcal/.cache/bazel/_bazel_tcal/69bdc7c11d7de775bee1f8b708726bda/external/rules_hdl/pdk/BUILD;
however, a source file of this name exists.  
(Perhaps add 'exports_files(["build_defs.bzl"])' to pdk/BUILD?) 
and referenced by '//xls/build_rules:xls_codegen_rules_bzl'

[QuantamHD]

Interesting. Didn't know that.