#23384 better logic in get_install_location().
Previously, this method would just return the location of the current
code. Now it attempts to locate the associated code correctly, first by
looking for a localized config, then by following the redirection files
defined in the PC for non-localized configs, and lastly - as a fallback
- it falls back on introspecting the currently running code.

Also removed two 'magical' legacy environment variables
TANK_CORE_LOCATION_OVERRIDE and TANK_INSTALL_LOCATION that are not being
used by the code base and were there to support legacy dev and
test setups.
#23382 added sgtk proxy
Tweak: removed double 'v' prefix for version numbers in api repr and str
Tweaks to api version mismatch copy. Fixed an issue with path lookups.