Fix extra-prog-path propagation in the codebase.

Extra prog paths were being handled in many different ways all thorugh
the codebase. This PR introduces a unified way to look at them.

Aiming for traceability, the addition of extra paths is now traced via
`logExtraProgramSearchPath`. All appearances of
`modifyProgramSearchPath` are replaced with `appendProgramSearchPath`
which traces the added paths.

`progInvokePathEnv` was only being set by GHC for some paths to
executables in components and only under certain circumstances. Now
every `ghcInvocation` sets the extra paths directly into `pkgInvokeEnv`.

In particular this fixes PATH issues when running MinGW cabal in
PowerShell, as usually for other OSes the system path contains most of
the expected directories.

(cherry picked from commit 46df8ba7)

# Conflicts:
#	Cabal/src/Distribution/Simple/Configure.hs
#	Cabal/src/Distribution/Simple/ConfigureScript.hs
#	Cabal/src/Distribution/Simple/GHC.hs
#	Cabal/src/Distribution/Simple/GHCJS.hs
#	Cabal/src/Distribution/Simple/Program/Db.hs
#	Cabal/src/Distribution/Simple/Program/Find.hs
#	Cabal/src/Distribution/Simple/Program/GHC.hs
#	Cabal/src/Distribution/Simple/Program/Run.hs
#	Cabal/src/Distribution/Simple/Program/Types.hs
#	cabal-install/src/Distribution/Client/CmdExec.hs
#	cabal-install/src/Distribution/Client/CmdInstall.hs
#	cabal-install/src/Distribution/Client/CmdRun.hs
#	cabal-install/src/Distribution/Client/Config.hs
#	cabal-install/src/Distribution/Client/Get.hs
#	cabal-install/src/Distribution/Client/HttpUtils.hs
#	cabal-install/src/Distribution/Client/ProjectConfig.hs
#	cabal-install/src/Distribution/Client/ProjectOrchestration.hs
#	cabal-install/src/Distribution/Client/ProjectPlanning.hs
#	cabal-install/src/Distribution/Client/SetupWrapper.hs
#	cabal-install/src/Distribution/Client/VCS.hs
#	cabal-install/tests/UnitTests/Distribution/Client/Get.hs
#	cabal-install/tests/UnitTests/Distribution/Client/VCS.hs