fix(rust_analyzer): resolve external crate sources via output_base, not execroot

[lsjostro]

Fixes the first bug described in #4057 (as requested by @UebelAndre there, this PR covers Bug 1 only; Bug 2 will get its own issue).

Problem

gen_rust_project emits external crate paths through the execution root:

{output_base}/execroot/_main/external/rules_rust+crate+anyhow-1.0.0/src/lib.rs

The execroot's external/ symlinks are ephemeral — they are only guaranteed to exist while a build action is running — so once the build finishes these paths dangle. rust-analyzer then cannot read any external crate sources: no completions, no go-to-definition, no inline docs for third-party dependencies (anyhow, serde, clap, …).

The stable location of external repository sources is {output_base}/external/.

Fix

In deserialize_with_substitution, rewrite {execution_root}/external/ prefixes to {output_base}/external/ after the existing template substitutions. bazel-out/ paths (generated files) are unaffected and stay under the execution root where they belong.

Testing

Added a unit test covering the four path shapes (external via __EXEC_ROOT__, generated file under bazel-out/, __OUTPUT_BASE__, and __WORKSPACE__); it fails without the fix and passes with it. We have also been running this fix as a patch on top of rules_rust in our bzlmod monorepo, where it restores full rust-analyzer IDE support for external crates.

Per the AI tools policy: this change was prepared with LLM assistance (see Assisted-by commit trailer), reviewed and tested by me.

[lsjostro]

Fresh datapoint on why this matters, re-measured today in our bzlmod monorepo (181 crates in rust-project.json across several crate closures):

Regenerating rust-project.json without this fix, 177 of the 181 crates — every third-party crate — get a root_module (plus CARGO_MANIFEST_DIR/CARGO_MANIFEST_PATH env) pointing into <output_base>/execroot/_main/external/...; 556 execroot path references in total. Those symlinks are rearranged by the next build and disappear on bazel clean, which breaks rust-analyzer's file→crate mapping (external crate items show up as unresolved imports).

With the fix, all 556 references resolve via <output_base>/external/ instead — zero execroot paths — and on Bazel 9 they canonicalize further to stable repo-contents-cache paths, so the project survives rebuilds and bazel clean.

(Measured against a current rules_rust pin carrying the same deserialize_with_substitution code; we ship this fix as a local patch in production in the meantime.)