Improve performance of package comment regex (#1799)

* Improve performance of package comment regex

By anchoring to the start of the document (since that's the only place
a package comment will appear), the number of false matches is greatly
reduced in some files.

Fixes #1747

* Simpler version with better tolerance of initial comments

Author: kalafut

syntax/go.vim

@@ -355,7 +355,7 @@ if go#config#HighlightBuildConstraints() || go#config#FoldEnable('package_commen
         \ . ' end=/\v\n\s*package/he=e-7,me=e-7,re=e-7'
         \ . ' contains=@goCommentGroup,@Spell'
         \ . (go#config#FoldEnable('package_comment') ? ' fold' : '')
-  exe 'syn region  goPackageComment    start=/\v\/\*.*\n(.*\n)*\s*\*\/\npackage/'
+  exe 'syn region  goPackageComment    start=/\v^\s*\/\*.*\n(.*\n)*\s*\*\/\npackage/'
         \ . ' end=/\v\*\/\n\s*package/he=e-7,me=e-7,re=e-7'
         \ . ' contains=@goCommentGroup,@Spell'
         \ . (go#config#FoldEnable('package_comment') ? ' fold' : '')