pantsbuild / pants / 37997622094

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Add `[golang].third_party_target_generation = imported` (closes #20340) (#23665)

## Background

Closes #20340. Implements the design @rdeknijf proposed and @benjyw
approved there; the design is theirs, this is our implementation of it,
opened with their go-ahead.

Adds `[golang].third_party_target_generation` = `build-list` (default,
today's behavior) | `imported`. Under `imported`,
`go_third_party_package` targets are generated only for third-party
packages reachable through the import graph from first-party code, so
target generation stops downloading and analyzing modules nothing
imports.

#23499's `third_party_target_granularity = module` does not cover this:
`generate_targets_from_go_mod` calls
`download_and_analyze_third_party_packages` unconditionally, and the
granularity branch selects from the already-resolved result. The two
compose — import scoping decides *which* modules are resolved,
granularity decides how many targets each produces.

Four parts: constraint-free `AllImports`/`AllTestImports` on the package
analyzer (so build-tag-gated, other-platform and `tools.go` imports
still resolve); a first-party root scan; a fixpoint walk downloading
only reached modules; and a codegen roots union with Protobuf
registered, so generated `.pb.go` imports still get targets.

The root scan reads `.go` files from the filesystem rather than
resolving targets. That is forced: its only caller is a target
generator, so touching the target graph deadlocks the engine with `The
dependency graph contained a cycle`. It is easy to miss, because
injecting `AllTargets` there leaves `pants list` green in this repo —
which has no `go_mod` targets — and only fails once a real one exists.

## Measured

32-`go_mod` production repo, `check src/go/config::` (an 8-module leaf),
fresh local store:

| scenario | build-list | imported |
| -- | -- | -- |
| remote cache off | 420s | 165s |
| remote cache on, warm | 446s | **96s** |

| | build-list | imported |
| -- | -- | -- ... (continued)

228 of 240 new or added lines in 7 files covered. (95.0%)

96260 of 103581 relevant lines covered (92.93%)

3.66 hits per line