include cleanup

The Zeek code base has very inconsistent #includes.  Many sources
included a few headers, and those headers included other headers, and
in the end, nearly everything is included everywhere, so missing
#includes were never noticed.  Another side effect was a lot of header
bloat which slows down the build.

First step to fix it: in each source file, its own header should be
included first to verify that each header's includes are correct, and
none is missing.

After adding the missing #includes, I replaced lots of #includes
inside headers with class forward declarations.  In most headers,
object pointers are never referenced, so declaring the function
prototypes with forward-declared classes is just fine.

This patch speeds up the build by 19%, because each compilation unit
gets smaller.  Here are the "time" numbers for a fresh build (with a
warm page cache but without ccache):

Before this patch:

 3144.94user 161.63system 3:02.87elapsed 1808%CPU (0avgtext+0avgdata 2168608maxresident)k
 760inputs+12008400outputs (1511major+57747204minor)pagefaults 0swaps

After this patch:

 2565.17user 141.83system 2:25.46elapsed 1860%CPU (0avgtext+0avgdata 1489076maxresident)k
 72576inputs+9130920outputs (1667major+49400430minor)pagefaults 0swaps

src/file_analysis/Manager.h

@@ -6,26 +6,30 @@
 #include <set>
 #include <map>
 
-#include "Dict.h"
+#include "Component.h"
 #include "Net.h"
-#include "Conn.h"
-#include "Val.h"
-#include "Analyzer.h"
-#include "Timer.h"
-#include "EventHandler.h"
 #include "RuleMatcher.h"
 
-#include "File.h"
-#include "FileTimer.h"
-#include "Component.h"
-#include "Tag.h"
 #include "plugin/ComponentManager.h"
-#include "analyzer/Tag.h"
 
 #include "file_analysis/file_analysis.bif.h"
 
+using std::map;
+using std::set;
+
+class TableVal;
+class VectorVal;
+
+namespace analyzer {
+class Analyzer;
+class Tag;
+}
+
 namespace file_analysis {
 
+class File;
+class Tag;
+
 /**
  * Main entry point for interacting with file analysis.
  */