formula_auditor: allow pkg-config alias for pkgconf migration

[cho-m]

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We've discussed migrating pkg-config to pkgconf a couple times, e.g. Homebrew/homebrew-core#194885

One of requirements is retaining compatibility with wide usage of depends_on "pkg-config" which seems to be best handled through an alias and this audit exception.

[MikeMcQuaid]

Good to merge once there's a comment!