amalgomate operation fails using Go 1.20

[nmiyake]

• Set local version of Go to be version 1.20
• Clone the repository github.com/palantir/godel-okgo-asset-compiles
• Run amalgomate with the arguments --config check.yml --output-dir generated_src --pkg amalgomatedcheck in that directory

What happened?

Failed with output:

Error: amalgomate failed for compiles: failed to repackage files specified in configuration: failed to rewrite imports for module &{Path:github.com/palantir/go-compiles Dir:/Volumes/git/go/src/github.com/palantir/godel-okgo-asset-compiles/vendor/github.com/palantir/go-compiles}: failed to walk directory /Volumes/git/go/src/github.com/palantir/godel-okgo-asset-compiles/generated_src/internal: failed to unmarshal JSON: go: finding module for package golang.org/x/tools
{
        "ImportPath": "golang.org/x/tools",
        "Match": [
                "golang.org/x/tools"
        ],
        "Incomplete": true,
        "Error": {
                "ImportStack": [],
                "Pos": "",
                "Err": "cannot query module due to -mod=vendor"
        }
}
: invalid character 'g' looking for beginning of value

What did you want to happen?

Operation should succeed (as it does when local Go version is set to 1.19).

[nmiyake]

This is due to a combination of 2 different issues caused by a change to go list in Go 1.20:

• cmd/go: list -json prints finding module for package even in vendor and readonly mode golang/go#58417: in Go 1.20, go list can include stderr output when listing a module in a vendor directory, and amalgomate currently parses CombinedOutput of the operation as JSON. This needs to be changed to only parse stdout.
• cmd/go: awkward "list -json" error reporting for a vendor subdirectory that does not contain a Go package golang/go#58418: the current amalgomate behavior depends on the "Dir" field being populated for a package lookup of a vendored module that is not a Go package. Although this happened to work in Go 1.19 and earlier, it's not actually a supported workflow and this output no longer exists in Go 1.20 (and will not be restored). The logic in amalgomate needs to be updated to deal with this scenario.