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.