feat: detect when go mod tidy and build scaffolding is needed

[matt2e]

closes #3129

Increases performance of go builds by 0.5s-0.8s in some cases.

Changes:

• Track if any thing has changed that would require us to
  • call go mod tidy (in either the main module directory, or in the build directory)
  • scaffold the build template (if we scaffold, we also need to run go mod tidy)
• If a build fails for any reason, reset ongoing build state so next time we always rebuild everything.

Trigger details:

• If we detect changes in go.mod, go.sum or types.ftl.go, always rebuild fully
• If the list of imported packages has changed, go mod tidy is required in the module's directory
• If the MainModuleContext has changed in any way, then the build template needs to be scaffolded, and go mod tidy needs to be run in .ftl/go/main

Performace:

• Scaffolding is super fast
• Running go mod tidy usually takes around 0.5-0.8s (but can take more if network requests are required)
• The 2 go mod tidy executions are run in parallel, so if either is required then there is no real performance gain. If neither is required then we see the performance improvement.
• The rest of the build time is split between extracting schema (0.5-1.5s) and compiling (0.5-1.5s)

[alecthomas]

Shouldn't we log this error?

[matt2e]

All errors returned from this function get logged by the build engine, so we don't need to double log here.