Makefile performance improvements

[danielrbradley]

Allow targets to be up-to-date by using the "sentinel file" pattern where each unit of work is tracked via a file in the .make directory to indicate when it was last successfully completed. Replace dependencies on phony targets with these sentinel files.

Other improvements:

• Add make schema target as a generic replacement for make tfgen.
• Split out a generate phase from all the build sdk targets to allow skipping running the SDK builds locally.
• Add make generate target for generating all things which need to be committed without having to wait on build steps.
• Add make prepare_local_workspace which currently encompases installing plugins and preparing upstream.
• Group .PHONY declarations near the definition of the phony target rather than in one very long line. This allows us to see if we're missing declarations for specific targets more easily.
• Fix the .pulumi/version getting out of sync with the installed files.
• Fix the clean target to include the bin and .make directories (remove the duplicate cleanup target alongside).
• Replace the broken help target which hasn't been functional in many years AFAIK.
• Move nuget setup for install_dotnet_sdk into the makefile from CI.
• Use the new standard targets (schema & provider) instead of the internal targets (tfgen_build_only, tfgen_no_deps, provider_no_deps)
• Save and restore makefile progress via artifacts (prerequisites.make and build_[language].make). This maintains file timestamps between jobs.

Commits are structued to address one target at a time for review and history.

Preview of new help text:

Usage: make [target]

Main Targets
  build (default)     Build the provider and all SDKs and install for testing
  generate            Generate all SDKs, documentation and schema
  provider            Build the local provider binary
  lint_provider<.fix> Run the linter on the provider (& optionally fix)
  test_provider       Run the provider tests
  test                Run the example tests (must run 'build' first)
  clean               Clean up generated files

More Precise Targets
  schema        Generate the schema
  generate_sdks Generate all SDKs
  build_sdks    Build all SDKs
  install_sdks  Install all SDKs
  provider_dist Build and package the provider for all platforms

Tool Targets
  ci-mgmt     Re-generate CI configuration from .ci-mgmt.yaml
  debug_tfgen Start a debug server for tfgen

Internal Targets (automatically run as dependencies of other targets)
  prepare_local_workspace  Prepare for building
  install_plugins          Install plugin dependencies
  upstream                 Initialize the upstream submodule, if present

Language-Specific Targets
  generate_[language]    Generate the SDK files ready for committing
  build_[language]       Build the SDK to check correctness
  install_[language]_sdk Install the SDK ready for testing

  [language] = nodejs python dotnet go java