Support `--excludeExternalTargets`

[jfirebaugh]

The external package no longer exists when --noenable_workspace is specified, causing bazel-differ to error:

ERROR: Skipping '//external:all-targets': no such package 'external': //external package is not available since the WORKSPACE file is disabled, please migrate to Bzlmod or temporarily enable WORKSPACE via --enable_workspace. See https://bazel.build/external/migration#bind-targets.

bazel-diff added the --excludeExternalTargets option to compensate. Tinder/bazel-diff#236 / Tinder/bazel-diff#246

[mmonaco]

Even better would be defaulting to auto-detect this.