Factor pkg_config_repository calls into bzl helpers

[jwnimmer-tri]

Relates #7259.

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This change is 

[jwnimmer-tri]

+@soonho-tri for all review per schedule, please.

I will test on macOS CI prior to merging, after #7733 lands first.

[soonho-tri]

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Reviewed 30 of 30 files at r1.
Review status: all files reviewed at latest revision, 1 unresolved discussion.

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WORKSPACE, line 47 at r1 (raw file):

# This local repository imports the protobuf build rules for Bazel (based on
# the upstream protobuf.bzl build rules); in constrast, the protobuf runtime is

I think this one (@libprotobuf) and the one below (@com_google_protobuf) were swapped by accident?

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Comments from Reviewable

[soonho-tri]

Review status: all files reviewed at latest revision, all discussions resolved.

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WORKSPACE, line 47 at r1 (raw file):

Previously, soonho-tri (Soonho Kong) wrote…

I think this one (@libprotobuf) and the one below (@com_google_protobuf) were swapped by accident?

Never mind. I just got confused.

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Comments from Reviewable

[jwnimmer-tri]

Review status: all files reviewed at latest revision, 1 unresolved discussion.

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WORKSPACE, line 47 at r1 (raw file):

Previously, soonho-tri (Soonho Kong) wrote…

Never mind. I just got confused.

OK Yeah, diff tools really struggle with PRs like this (I have a few more in the queue that are similarly scattered.)

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Comments from Reviewable

[jwnimmer-tri]

@drake-jenkins-bot mac-sierra-clang-bazel-experimental please
@drake-jenkins-bot mac-sierra-clang-cmake-experimental-matlab please
@drake-jenkins-bot mac-highsierra-clang-bazel-experimental-everything please
@drake-jenkins-bot mac-highsierra-clang-cmake-experimental please