fix(cli): rewrite 'doom sync'; deprecate 'doom build'

This changes 'doom sync' to be smarter about responding to changed
package recipes/pins, changes in Emacs version, or instances where the user
has copied a config to a new system.

In all these cases, the user would formerly have to know about a
specific combination of 'doom sync -u' and 'doom build' to ensure Doom
is in a good state. With this change, 'doom sync' handles all these
cases.

Also, 'doom build' is now deprecated (and 'doom sync' now has a
--rebuild option to mimic its old behavior).

Also also, sometimes, a package may silently fail when cloned (which
used to result in an empty repo). Now, if this is detected, cloning will
be re-attempted up to 3 times before aborting with much more visible
error.

Note: these are stopgap solutions, until v3 is finished.

README.md

@@ -138,8 +138,6 @@ commands you should know about:
 + `doom env` to dump a snapshot of your shell environment to a file that Doom
   will load at startup. This allows Emacs to inherit your `PATH`, among other
   things.
-+ `doom build` to recompile all installed packages (use this if you up/downgrade
-  Emacs).
 
 
 # Roadmap

bin/doom

@@ -280,7 +280,6 @@ SEE ALSO:
     ;; (load! "nuke" dir)
     ;; (load! "package" dir)
     ;; (load! "profile" dir)
-    ;; (defcli-obsolete! ((build b))   (sync "--rebuild") "v3.0.0")
     )
 
   (defcli-group! "Diagnostics"

docs/faq.org

@@ -402,14 +402,13 @@ This command is never needed for changes to =$DOOMDIR/config.el=.
 
 ** Copy or sync my config to another system?
 *Short answer:* it is safe to sync =$DOOMDIR= across systems, but not
-=$EMACSDIR=. Once moved, use ~$ doom sync && doom build~ to ensure everything is
-set up correctly.
+=$EMACSDIR=. Once moved, use ~$ doom sync~ to ensure everything is set up
+correctly.
 
 *Long answer:* packages can contain baked-in absolute paths and non-portable
 byte-code. It is never a good idea to mirror it across multiple systems, unless
 they are all the same (same OS, same version of Emacs, same paths). Most issues
-should be solved by running ~$ doom sync && doom build~ on the other end, once
-moved.
+should be solved by running ~$ doom sync~ on the other end, once moved.
 
 ** Start over, in case something went terribly wrong?
 Delete =$EMACSDIR/.local/straight= and run ~$ doom sync~.