feat: use PackageClauseOnly for parsing pkg name

[harry-hov]

Previously, we were parsing the whole file using MustParseFile(), which uses Go2Gno(), and incurred unnecessary overhead. This PR modifies the func PackageNameFromFileBody() to use mode PackageClauseOnly, which extracts the package name without the need for heavier parsing operations.

Contributors Checklist

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests
• Added new benchmarks to generated graphs, if any. More info here.

[zivkovicmilos]

Looks good 💯

Left some minor comments

[moul]

I suggest favoring parsing over regex, even though regex may have optimization benefits in some cases.

For the future, I recommend building a (lazy?) caching system (around memFile?). This could involve parsing and caching the file lazily, allowing specialized helper functions to navigate the AST and avoid double parsing.