Rollup merge of #154823 - jakubadamw:spdx-rs-replacement, r=Mark-Simulacrum

Replace the spdx-rs dependency with a minimal in-tree SPDX tag-value parser

The spdx-rs crate [is no longer maintained](https://github.com/doubleopen-project/spdx-rs/pulls) and is behind on its own dependency updates. It is currently used in [the collect-license-metadata tool](https://github.com/rust-lang/rust/tree/main/src/tools/collect-license-metadata), employing a single function therefrom: `spdx_rs::parsers::spdx_from_tag_value`, which parses the output of the `reuse` tool to extract file names, licences and copyright text.

This PR replaces the use of said function with a small minimal parser that handles just the subset of the SPDX tag-value format that is needed: `Tag: Value` line pairs and multi-line `<text>...</text>` blocks.

Coincidentally, this gets rid of the last transitive dependency on syn v1.

Author: JonathanBrouwer