fix: detect go package clause after leading comments

[mokevnin]

Problem

GO_PACKAGE_RE is anchored at the start of the snippet, not per line. A go block that opens with a comment therefore looks like a snippet without a package clause, and mdsf prepends its temporary package:

```go
// main.go
package main

func main() {}
```

gofmt then sees two package clauses and fails with expected declaration, found 'package'. Since error running gofmt exits zero, the block is neither formatted nor reported as unformatted — it just silently drops out of the check. Both a file-name comment and a //go:build constraint hit this, and both are common in documentation.

Change

A package clause is the first token of a Go file that is neither whitespace nor a comment, so has_go_package skips leading // and /* */ comments and applies the existing anchored pattern to the remainder. GO_PACKAGE_RE keeps its original semantics, and the TODO: check for multiline comments above it is resolved: a block comment containing a line that starts with package is not mistaken for a package clause.

Checks

cargo test -p mdsf --lib parser passes, with a new test_has_go_package module covering both directions (leading file-name comment, //go:build, a block comment before a real package clause, and the same block comment without one). cargo clippy --all-targets is clean. On a corpus of Go-heavy markdown, the block above goes from error running gofmt to being formatted normally.

[ccoVeille]

This enable multine matching, won't this break some use cases.

package is a reserved word in Go, so it sounds safe

But you may something odd like this snippet without a package

/*

I like my
package manager
*/

const foo = 1

But I feel like there is no better solution without dealing with complicated regexp to remove comments

[mokevnin]

Thanks, that example is a real hole and it pushed the fix to the right place.

Dropped the (?m) and matched Go's own rule instead: the package clause is the first token that is neither whitespace nor a comment. has_go_package skips leading // and /* */ and applies the existing anchored pattern to the remainder, so your snippet stays without a package clause and still gets the temporary one. No comment-removing regex needed, and the TODO: check for multiline comments above GO_PACKAGE_RE is gone with it.

Both directions are covered in a new test_has_go_package module, your snippet included.