Various optimisations to PackageInfo reading and bug fixes.

* PackageInfo objects are now cached because NimScript evaluation is expensive.
* before/after hooks now return `true` by default.
* Bugfix: when hooks weren't found Nimble would still think that a hook told it
  to skip an action.
* PackageInfo now includes info about which hooks are defined to prevent
  unnecessary execution of those hooks.
* Probably more.

src/nimblepkg/nimscriptapi.nim

@@ -27,12 +27,13 @@ proc requires*(deps: varargs[string]) =
 template before*(action: untyped, body: untyped): untyped =
   ## Defines a block of code which is evaluated before ``action`` is executed.
   proc `action Before`*(): bool =
-    result = false
+    result = true
     body
 
 template after*(action: untyped, body: untyped): untyped =
   ## Defines a block of code which is evaluated after ``action`` is executed.
   proc `action After`*(): bool =
+    result = true
     body
 
 template builtin = discard