[roll] Roll fuchsia [check-licenses] Refactor readme generators to use direct initialization

This change removes the `builder.go` string-synthesis engine and
refactors the ecosystem-specific readme generators (`dart_pkg.go`,
`golib.go`, `rust_crate.go`) to directly instantiate and populate the
`Readme` struct.

Previously, these generators would parse their respective package
manifests (e.g., Cargo.toml), use the builder to generate a massive
multi-line string formatted like a physical `README.fuchsia` file,
and then feed that string back into the `readme.go` text parser to
reconstruct a `Readme` struct.

This change eliminates that inefficient loop. The ecosystem parsers
now directly populate the `Readme` fields (Name, URL, Version,
Licenses), significantly reducing string allocations and simplifying
the architecture.

Additionally, the license loading logic was extracted from `NewReadme`
into a shared `loadLicenseFiles()` method to support this direct
initialization pattern.

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1567838
Original-Revision: 7746dfe1e9b626fdb073bebf12b42dcfc250cfb2
GitOrigin-RevId: 692093f5a347834db605cd21d1d96befd78daeed
Change-Id: Ifd09b38d8e6aabb663efa730514be6e050753804