refactor(buildcontext): unify Earthfile parsing into single pass and …

[janishorsts]

Previously, resolving an Earthfile required parsing the file twice: once through ParseVersion (via parseFeatures) to inspect feature flags, and later through earthfile.ParseFile to construct the AST. To bridge these separate passes, Resolver maintained its own parseCache alongside the existing buildFileCache.
Caching parsed ASTs to avoid re-parsing files that were already being cached by the resolver was an unnecessary layer of indirection.
This change unifies Earthfile parsing into a single pass:

1. newEarthfileBuild reads and parses the Earthfile into its full AST (earthfile.Tree) up front.
2. Feature flags are extracted directly from tree.Version, processed for warnings, and merged with CLI overrides.
3. The resulting buildFile struct carries both the feature configuration and the AST tree.
4. parseFeatures.go, version.go, and the redundant parseCache are removed.
   Parsing once and holding the result simplifies control flow, removes ~320 lines of redundant parsing logic, and keeps resolver caching in one clear place.

[github-actions]

➖ Are we earthbuild yet?

No change in "earthly" occurrences

📈 Overall Progress

Branch	Total Count
main	4050
This PR	4050
Difference	+0

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Keep up the great work migrating from Earthly to Earthbuild! 🚀

💡 Tips for finding more occurrences

Run locally to see detailed breakdown:

./.github/scripts/count-earthly.sh

Note that the goal is not to reach 0.
There is anticipated to be at least some occurences of earthly in the source code due to backwards compatibility with config files and language constructs.