Merge mustache between fbcode and xplat

Summary:
Demonstrate feasibility of merging fbcode and xplat copies of thrift on a single target, `thrift/compiler/detail/mustache:mustache`. To this end:

* Switch fbcode_builder from fbcode to xplat.
* Map dependencies to the correct variants for fbcode.
* Exclude mustache directory from target remapping. Once the merge is complete remapping rules will no longer be necessary at all, further reducing maintenance burden and complexity.
* Remove one copy of mustache (~2.6kLOC).

This also demonstrates that autodeps are working as expected after the fix in D67919546.

Reviewed By: yoney

Differential Revision: D67676056

fbshipit-source-id: ba3854c0997c4dd10f47a5e623381c3dadef4ecc

Author: vitaut

build/fbcode_builder/manifests/fbthrift

@@ -1,6 +1,6 @@
 [manifest]
 name = fbthrift
-fbsource_path = fbcode/thrift
+fbsource_path = xplat/thrift
 shipit_project = fbthrift
 shipit_fbcode_builder = true
 
@@ -37,11 +37,11 @@ xxhash
 python
 
 [shipit.pathmap]
-fbcode/thrift/public_tld = .
-fbcode/thrift = thrift
+xplat/thrift/public_tld = .
+xplat/thrift = thrift
 
 [shipit.strip]
-^fbcode/thrift/thrift-config\.h$
-^fbcode/thrift/perf/canary.py$
-^fbcode/thrift/perf/loadtest.py$
-^fbcode/thrift/.castle/.*
+^xplat/thrift/thrift-config\.h$
+^xplat/thrift/perf/canary.py$
+^xplat/thrift/perf/loadtest.py$
+^xplat/thrift/.castle/.*