Move the code related to plugin loading / installation to "flipper-plugin-lib"

Summary:
Sorry for so long diff, but actually there are no functional changes, just refactoring to make further changes of Plugin Manager easier to understand.

I've de-coupled the code related to plugin management from UI code and moved it from PluginInstaller UI component (which will be replaced soon by new UI) to "flipper-plugin-lib".  So pretty much everything related to plugin discovery and installation now consolidated in this package.

Additionally, this refactoring enables re-using of plugin management code in "flipper-pkg", e.g. to create CLI command for plugin installation from NPM, e.g.: `flipper-pkg install flipper-plugin-reactotron`.

Reviewed By: passy

Differential Revision: D23679346

fbshipit-source-id: 82e7b9de9afa08c508c1b228c2038b4ba423571c

desktop/types/npm-api.d.ts

@@ -32,5 +32,6 @@ declare module 'npm-api' {
   export interface Package {
     name: string;
     version: string;
+    [name: string]: string;
   }
 }