vcs/source: strip Go major version segment from symbol file paths

[simonswine]

Summary

Go module paths for major versions ≥ 2 include a version suffix (e.g. github.com/grafana/pyroscope/v2/pkg/foo.go). When resolving source files for VCS linking, the code strips the GitHub repo prefix — but this leaves a v2/ segment that doesn't exist as a directory in the GitHub repo, causing file lookups to fail.

This PR strips the major version path element (any vN/ segment immediately after the repo prefix) so that source file lookups work correctly for symbols from v2+ binaries.

Test plan

• New test cases path_with_relative_repository_prefix_and_major_version and path_with_absolute_repository_prefix_and_major_version in find_go_test.go cover the fix
• All existing Test_tryFindGoFile cases continue to pass

---

Note

Medium Risk
Adjusts path normalization in VCS source lookup, which could change which files are fetched for some repos (e.g., if a real v2/ directory exists). Coverage is added for common v2/v1 cases, reducing regression risk.

Overview
Fixes Go source-file resolution for module paths that include a major version suffix by stripping a leading vN/ segment (only for N >= 2) after the repo prefix in tryFindGoFile.

Adds targeted tests ensuring v2/ paths (absolute and relative) resolve to the repo root file, while a legitimate v1/ directory is not stripped.

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[marcsanmi]

LGTM

[marcsanmi]

Nit: Are isDigits and Atoi redundant? e.g

if seg := path[:i]; len(seg) > 1 && seg[0] == 'v' {
    if majorVer, err := strconv.Atoi(seg[1:]); err == nil && majorVer >= 2 {
        path = path[i+1:]
    }
}

[aleks-p]

I guess there could be repos that use v2 or v3 directories for versioning, instead of tags

https://go.dev/ref/mod#vcs-version

If a module is defined in the repository root directory or in a major version subdirectory of the root directory, then each version tag name is equal to the corresponding version.

Not sure how common that is, we could do a fetch with it stripped and another one without the strip if we get a 404 - or we could wait for this to get reported as a real problem.

[simonswine]

I don't think this is a correct fix, might not even be a problem. At least for loki with vendor we would break that.

Let's revisit this later