SF.12: Rewrite to eliminate the notions of "project" and "component"; also copyedit.

The major change here is simply to delete parentheticals that muddy the waters
by talking about files belonging to "projects" or "components". My experience
has been that in industry we have "codebases," which are sometimes split up into
"repositories" and/or "libraries" (which may be installed in e.g. /usr/local/include
or may be used straight from their source tree), and everyone works on everything
or at least knows someone who works on the other thing. Figuring out what's
"in the same project" and what's "in a different project" is often fuzzy.

I would try to set up the build system so that its `target_include_directories`
(or equivalent) could find every header via `<>`, and then use `<>` everywhere:
even for files in the "same" component. (This does require that you follow SF.12's
advice for "Library creators", though, which I admit I don't for just about all
of my hobby projects.)

Strangely, the "Enforcement" section seems to have been written by a `<>` partisan
like me, even though the rule itself is `""`-endian.
I copyedited the "Enforcement" section for grammar; did I unconsciously misinterpret
what it was trying to say? What *was* it trying to say? Should it just be deleted?

Author: Quuxplusone

Diff for: CppCoreGuidelines.md

@@ -19298,37 +19298,37 @@ the two forms of `#include` selected using the angle (`<>`) or quoted (`""`) syn
 advantage of this and use different search algorithms and methods for specifying the include path.
 
 Nevertheless, the guidance is to use the quoted form for including files that exist at a relative path
-to the file containing the `#include` statement (from within the same component or project) and to use
+to the file containing the `#include` statement and to use
 the angle bracket form everywhere else, where possible. This encourages being clear about the locality
 of the file relative to files that include it, or scenarios where the different search algorithm is
-required. It makes it easy to understand at a glance whether a header is being included from a local
-relative file versus a standard library header or a header from the alternate search path (e.g. a header
-from another library or a common set of includes).
+required. It makes it easy to understand at a glance whether a header is being included from a locally
+relative file, versus a standard library header or a header from the header search path (e.g. a header
+from another library or from a common set of includes).
 
 ##### Example
 
     // foo.cpp:
     #include <string>                // From the standard library, requires the <> form
-    #include <some_library/common.h> // A file that is not locally relative, included from another library; use the <> form
-    #include "foo.h"                 // A file locally relative to foo.cpp in the same project, use the "" form
-    #include "foo_utils/utils.h"     // A file locally relative to foo.cpp in the same project, use the "" form
-    #include <component_b/bar.h>     // A file in the same project located via a search path, use the <> form
+    #include <some_library/common.h> // A file that is not locally relative, included from another library: use the <> form
+    #include "foo.h"                 // A file located relative to foo.cpp: use the "" form
+    #include "foo_utils/utils.h"     // A file located relative to foo.cpp: use the "" form
+    #include <component_b/bar.h>     // A file located via the header search path: use the <> form
 
 ##### Note
 
-Failing to follow this results in difficult to diagnose errors due to picking up the wrong file by
+Failing to follow this results in difficult-to-diagnose errors due to picking up the wrong file by
 incorrectly specifying the scope when it is included. For example, in a typical case where the `#include ""`
-search algorithm may search for a file existing at a local relative path first, then using this form to refer
-to a file that is not locally relative could mean that if a file ever comes into existence at the
-local relative path (e.g. the including file is moved to a new location), it will now be found ahead
+search algorithm may search for a file existing at a locally relative path first, then using this form to refer
+to a file that is not locally relative could mean that if a file by that name ever comes into existence at the
+locally relative path (e.g. the including file is moved to a new location), it will now be found ahead
 of the previous include file and the set of includes will have been changed in an unexpected way.
 
-Library creators should put their headers in a folder and have clients include those files using the
-relative path `#include <some_library/common.h>`
+Library creators should put their headers in a folder and have clients include those files using
+angle brackets: `#include <some_library/common.h>`.
 
 ##### Enforcement
 
-A test should identify headers referenced via `""` could be referenced with `<>`.
+A tool could identify headers referenced via `""` that could have been referenced with `<>`.
 
 ### <a name="Rs-namespace"></a>SF.20: Use `namespace`s to express logical structure