feat: replace input package references in stream definitions

[teresaromero]

Replace input package references with resolved input types

Closes elastic#3380

Composable packages stack — part 4 of 4 (final).
This PR is stacked on elastic#3329 (closes elastic#3278), #1 (closes elastic#3279), and #2 (closes elastic#3280). Merge order: elastic#3329 → #1 → #2 → this PR.
Each PR will be rebased onto main once its predecessor merges.

Summary

Final step of the composable-packages build pipeline: replaces all package: <input-pkg-name> references in policy template inputs and data stream stream entries with the actual input type identifier (e.g. logfile) resolved from the required input package's manifest.

This step runs after variable merging (which relies on stream.Package / input.Package to identify entries to process), so ordering matters.

After this transform the resulting manifests are indistinguishable from hand-authored integration package manifests — the package: key is removed entirely.

What changed

internal/requiredinputs/streamdefs.go (new, ~190 lines)

• ReplaceStreamPackageRefs: resolves input type from required input package metadata and rewrites both policy_templates[].inputs[] and data_stream/*/manifest.yml → streams[] entries.
• Falls back to input package title/description when the integration does not declare its own.

internal/requiredinputs/streamdefs_test.go (new, ~460 lines)

• Covers: policy template input replacement, data stream stream replacement, title/description fallback, multi-stream packages.

How to test

The existing test/manual_packages/required_inputs/with_input_package_requires fixture exercises this path end-to-end:

cd test/manual_packages/required_inputs/test_input_pkg
elastic-package build

elastic-package stack up --version 9.4.0-SNAPSHOT

cd test/manual_packages/required_inputs/with_input_package_requires
elastic-package build
# Inspect build/: policy_templates[].inputs[].type and data_stream streams[].type should now
# contain the resolved input type (e.g. "logfile"); no "package:" key should remain.

Dependencies

• bundle templates of required input packages elastic/elastic-package#3329, feat: merge variables from required input packages into composable manifests #1, and feat: bundle field definitions from required input packages #2 must merge first.