feat: add odin_library rule for collection-based package imports

[clappingmonkey]

What does this PR do?

Adds the rule — a source-aggregation rule that groups files and exposes them via so that targets can import them as Odin collections.

New files

• odin/private/common.bzl — shared get_package_dir() helper (extracted from binary.bzl)
• odin/private/library.bzl — OdinLibraryInfo provider + odin_library rule
• e2e/smoke/lib/greetings.odin — library test source

Modified files

• odin/private/binary.bzl — added deps attr, -collection:<name>=<root> flags, dep srcs in action inputs, duplicate collection name validation
• odin/defs.bzl — exports odin_library
• e2e/smoke/BUILD.bazel — odin_library target + deps on odin_binary
• e2e/smoke/hello/hello.odin — imports library via import greetings "greetings:lib"
• README.md — documents odin_library + deps attr

Design

• odin_library is source aggregation only (no compilation step)
• Collection name = the label name of the odin_library target
• Collection root = parent directory of the package directory
• odin_binary consumes deps by passing -collection:<name>=<root> to the Odin compiler
• Odin uses the directory name as the default import alias (not the package declaration name), so users should use import myname "collection:dir" for explicit aliases

Edge cases handled

• Empty deps → backward compatible (existing odin_binary works unchanged)
• Duplicate collection names across deps → fail() with clear error
• Root-level srcs (no parent dir) → fail() with clear error
• Lockfile churn excluded from this PR

How was it tested?

• bazel build //... in root workspace — passes
• cd e2e/smoke && bazel build //... — passes
• ./e2e/smoke/bazel-bin/hello — prints both "Hellope, Odin from Bazel!" and "Greetings from the library, Bazel!"
• pre-commit run --all-files — buildifier + typos pass
• Adversarial review by @change-validator — all 3 findings addressed (duplicate collection names, root-level srcs rejection, unused attrs removed)

Checklist

• PR title follows conventional commit format
• CI passes (check.yaml — lint + test matrix)
• Tests: e2e smoke test updated with library dependency
• Documentation: README updated with odin_library rule + deps attr
• No breaking changes (existing odin_binary without deps works unchanged)