feat(developer): PackageCompiler and WindowPackageInstallerCompiler now implement KeymanCompiler

Relates to #9473.

Refactors the public API of PackageCompiler and
WindowsPackageInstallerCompiler to meet KeymanCompiler, including
asyncing a bunch of functions, and moving file write responsibilities
into the classes themselves.

The classes were updated together in a single commit because
WindowsPackageInstallerCompiler depends on PackageCompiler, and needed
refactoring for the updated API access there anyway.

Most of the test cases needed only minor patching, but introducing the
`async init()` function has caused them to all be async. The
test-messages module has a slight functional change with the compile
process running completely rather than stopping on first message, which
meant we needed to add extra dummy fixtures to avoid other errors that
arose later in the compile process. The keyman.exe fixture is a text
file, not a Windows executable.