enhancement: optimise local fast-path

[myitcv]

We can optimise the path where -g is not provided.

There are a few nice edge cases in this, hence why it's not implemented from the off 😄

Notes:

• The main package might not be the module path, so we need to search up the path for the module
• Need to encode main package paths
• Search the gobin cache for the provided main package
• If that fails, search $GOPATH/pkg/mod
• Need to do version resolution, including "latest" semantics