This is the flow of the compiler detection logic:

External to MSCommon:

  The Tool init modules, in their exists() routines, call -> msvc_exists(env)

At the moment, those modules are:
  SCons/Tool/midl.py
  SCons/Tool/mslib.py
  SCons/Tool/mslink.py
  SCons/Tool/msvc.py
  SCons/Tool/msvs.py

env may contain a version request in MSVC_VERSION, but this is not used
in the detection that follows from msvc_exists(), only in the later
batch that starts with a call to msvc_setup_env().

Internal to MSCommon/vc.py:

+ MSCommon/vc.py:msvc_exists:
| vcs = cached_get_installed_vcs(env)
| returns True if vcs > 0
|
+-> MSCommon/vc.py:cached_get_installed_vcs:
  | checks global if we've run previously, if so return it
  | populate the global from -> get_installed_vcs(env)
  |
  +-> MSCommon/vc.py:get_installed_vcs:
    | loop through "known" versions of msvc, granularity is maj.min
    |   check for product dir -> find_vc_pdir(env, ver)
    |
    +-> MSCommon/vc.py:find_vc_pdir:
      | From the msvc-version to pdir mapping dict, get reg key base and value
      | If value is none -> find_vc_pdir_vswhere(ver, env)
      |
      +-> MSCommon/vc.py:find_vc_pdir_vswhere:
        | From the vc-version to VS-version mapping table get string
        | Figure out where vswhere is -> msvc_find_vswhere()
        | Use subprocess to call vswhere, return first line of match
        /
      | else get product directory from registry (<= 14.0)
      /
    | if we found one -> _check_cl_exists_in_vc_dir(env, pdir, ver)
    |
    +-> MSCommon/vc.py:_check_cl_exists_in_vc_dir:
      | Figure out host/target pair
      | if version > 14.0 get specific version by looking in
      |    pdir + Auxiliary/Build/Microsoft/VCToolsVersion/default.txt
      |    look for pdir + Tools/MSVC/{specver}/bin/host/target/cl.exe
      | if 14.0 or less, "do older stuff"

All of this just got us a yes-no answer on whether /some/ msvc version
exists, but does populate __INSTALLED_VCS_RUN with all of the top-level
versions as noted for get_installed_vcs

Externally:

  Once a module's exists() has been called (or, in the case of
  clang/clangxx, after the compiler has been detected by other means -
  those still expect the rest of the msvc chain but not cl.exe)
  the module's generate() function calls -> msvc_setup_env_once(env)

Internally:

+ MSCommon/vc.py:msvc_setup_env_once:
| checks for environment flag MSVC_SETUP_RUN
| if not, -> msvc_setup_env(env) and set flag
|
+-+ MSCommon/vc.py:msvc_setup_env:
  | set ver from -> get_default_version(env)
  |
  +-+ MSCommon/vc.py:get_default_version:
    | if no version specified in env.MSVC_VERSION:
    |   return first entry from -> cached_get_installed_vcs(env)
    | else return requested version
    /
  | get script from MSVC_USE_SCRIPT if set to a filename
  | -> script_env(script)
  |
  +-+ MSCommon/vc.py:script_env:
    | return (possibly cached) script variables matching script arg
    /
  | else -> msvc_find_valid_batch_script(env, version)
  |
  +-+ MSCommon/vc.py:msvc_find_valid_batch_script:
    | Build a list of plausible target values, and loop through
    |   look for host + target -> find_batch_file(env, ver, host, target)
    |
    +-+ MSCommon/vc.py:find_batch_file:
      | call -> find_vc_pdir (see above)
      | use the return to construct a version-biased batfile path, check
      /
    | if not found, try sdk scripts (unknown if this is still useful)


Problems:
- For VS >= 2017, VS and VS are not 1:1, there can be many VC for one VS
- For vswhere-ready versions, detection does not proceed beyond the
  product level ("2019") into individual "features" (individual msvc)
- As documented for MSVC_VERSION, compilers can only be requested if versions
  are from the set in _VCVER, so 14.1 but not 14.16 or 14.16.27023
- Information found in the first pass (msvs_exists) isn't really
  available anywhere except the cached version list, since we just
  return true/false.
- Since msvc_exists chain of calls does not look at version, we
  can proceed to compiler setup if *any* msvc was found, even if the
  one requested wasn't found.