feat(vet): accept -E/--external to pass external KCL packages

[Peefy]

Fixes kcl-lang/kcl#1877.

kcl vet did not expose any way to declare external KCL package
mappings, so schemas that imported other packages failed validation
even though kcl run handled the same scenario via -E/--external
and the underlying Rust service already supported external_pkgs.

Changes

• Add the same -E/--external flag used by kcl run to kcl vet.
  Each value is parsed as pkg_name=pkg_path and forwarded to the
  embedded validate.ValidateOptions.ExternalPackages (newly added
  in kcl-lang/kcl-go alongside this change).
• Surface malformed entries (pkg_name or pkg_path missing) as a
  clear error rather than silently dropping them, matching the
  long-standing behaviour of kcl run --external consumers.
• Add a vet_test.go covering the flag registration, the parser,
  and the gRPC conversion helper.

Test plan

• go test ./cmd/kcl/commands/... -run Vet — passes locally,
  including all eight table-driven parser cases, the flag
  registration check, and the empty-field short-circuit.
• Manual end-to-end:

# ext/kcl.mod + ext/main.k defines schema Value
# with_ext/schema.k imports ext
kcl vet -E ext=../ext data.json schema.k
# -> "Validate success!"

Without -E the historical "Cannot find the module" error is
preserved, so this change is strictly additive for existing
invocations.

Depends on kcl-lang/kcl-go#566, which adds the new
ValidateOptions.ExternalPackages field this CLI change forwards
into.

[coveralls]

Coverage Report for CI Build 31554848354

Coverage remained the same at 27.691%

Details

• Coverage remained the same as the base build.
• Patch coverage: No coverable lines changed in this PR.
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	892
Covered Lines:	247
Line Coverage:	27.69%
Coverage Strength:	0.3 hits per line

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💛 - Coveralls