Fix loading packages with Go modules disabled

[n-peugnet]

While packaging the v2 of this library in Debian, most of the tests were failing. This is because we use the GOPATH mode (so with Go modules disabled) to build and test packages.

This allows the parser to load packages even when Go modules are disabled, and when GOPATH mode is used.

I am not sure if this is the best way to handle this, but at least it works with Go modules either disabled or enabled. I don't really know either why the PkgPath looks like this in GOPATH mode, I didn't find in golang.org/x/tools's code where exactly this string is constructed from a quick search. So feel free to suggest a better way to solve this.

For the record, here is the kind of error that we got from running the tests in GOPATH mode:

=== RUN   TestSnippetWriter
    snippet_writer_test.go:33: unexpected error: error(s) in "_/home/nicolas/debian/golang-github-kubernetes-gengo/golang-github-kubernetes-gengo/obj-x86_64-linux-gnu/src/k8s.io/gengo/v2/generator/testdata/snippet_writer":
        -: cannot find package "_/home/nicolas/debian/golang-github-kubernetes-gengo/golang-github-kubernetes-gengo/obj-x86_64-linux-gnu/src/k8s.io/gengo/v2/generator/testdata/snippet_writer" in any of:
        	/usr/lib/go-1.24/src/_/home/nicolas/debian/golang-github-kubernetes-gengo/golang-github-kubernetes-gengo/obj-x86_64-linux-gnu/src/k8s.io/gengo/v2/generator/testdata/snippet_writer (from $GOROOT)
        	/home/nicolas/debian/golang-github-kubernetes-gengo/golang-github-kubernetes-gengo/obj-x86_64-linux-gnu/src/_/home/nicolas/debian/golang-github-kubernetes-gengo/golang-github-kubernetes-gengo/obj-x86_64-linux-gnu/src/k8s.io/gengo/v2/generator/testdata/snippet_writer (from $GOPATH)
--- FAIL: TestSnippetWriter (0.02s)
FAIL
FAIL	k8s.io/gengo/v2/generator	0.033s
[...]
=== RUN   TestFindPackages
    parse_test.go:161: wrong pkgs:
        want: [
          "k8s.io/gengo/v2/parser/testdata/root1",
          "k8s.io/gengo/v2/parser/testdata/root2",
          "k8s.io/gengo/v2/parser/testdata/roots345/root3",
          "k8s.io/gengo/v2/parser/testdata/roots345/root3/lib3",
          "k8s.io/gengo/v2/parser/testdata/roots345/root4",
          "k8s.io/gengo/v2/parser/testdata/roots345/root4/lib4",
          "k8s.io/gengo/v2/parser/testdata/roots345/root5",
          "k8s.io/gengo/v2/parser/testdata/roots345/root5/lib5"
        ]
        got:  [
          "_/home/nicolas/debian/golang-github-kubernetes-gengo/golang-github-kubernetes-gengo/obj-x86_64-linux-gnu/src/k8s.io/gengo/v2/parser/testdata/root1",
          "_/home/nicolas/debian/golang-github-kubernetes-gengo/golang-github-kubernetes-gengo/obj-x86_64-linux-gnu/src/k8s.io/gengo/v2/parser/testdata/root2",
          "_/home/nicolas/debian/golang-github-kubernetes-gengo/golang-github-kubernetes-gengo/obj-x86_64-linux-gnu/src/k8s.io/gengo/v2/parser/testdata/roots345/root3",
          "_/home/nicolas/debian/golang-github-kubernetes-gengo/golang-github-kubernetes-gengo/obj-x86_64-linux-gnu/src/k8s.io/gengo/v2/parser/testdata/roots345/root3/lib3",
          "_/home/nicolas/debian/golang-github-kubernetes-gengo/golang-github-kubernetes-gengo/obj-x86_64-linux-gnu/src/k8s.io/gengo/v2/parser/testdata/roots345/root4",
          "_/home/nicolas/debian/golang-github-kubernetes-gengo/golang-github-kubernetes-gengo/obj-x86_64-linux-gnu/src/k8s.io/gengo/v2/parser/testdata/roots345/root4/lib4",
          "_/home/nicolas/debian/golang-github-kubernetes-gengo/golang-github-kubernetes-gengo/obj-x86_64-linux-gnu/src/k8s.io/gengo/v2/parser/testdata/roots345/root5",
          "_/home/nicolas/debian/golang-github-kubernetes-gengo/golang-github-kubernetes-gengo/obj-x86_64-linux-gnu/src/k8s.io/gengo/v2/parser/testdata/roots345/root5/lib5"
        ]
    parse_test.go:186: wrong pkgs:
        want: ["k8s.io/gengo/v2/parser/testdata/does-not-parse"]
        got:  [
          "_/home/nicolas/debian/golang-github-kubernetes-gengo/golang-github-kubernetes-gengo/obj-x86_64-linux-gnu/src/k8s.io/gengo/v2/parser/testdata/does-not-parse"
        ]
--- FAIL: TestFindPackages (0.04s)
[...]
FAIL	k8s.io/gengo/v2/parser	0.297s

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• ✅login: n-peugnet / (2d36c62)

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