Iterating all options and potential file system access is not great for
every plugin lookup request. Instead, lazily create a single lookup table
keyed by module name.
'load-plugin-library', 'load-plugin-executable', '-plugin-path' and
'-external-plugin-path' should be searched in the order they are
specified in the arguments.
Previously, for example '-plugin-path' used to precede
'-external-plugin-path' regardless of the position in the arguments.
* Factor out ASTContext plugin loading to newly introduced 'PluginLoader'
* Insert 'DependencyTracker' to 'PluginLoader'
* Add dependencies right before loading the plugins
rdar://104938481
