Refactor .buckconfig common elements into .buckconfig.d

Summary:
Move common settings, such as `prelude = bundled` and shim aliases, into a
`common.buckconfig` file inside the `.buckconfig.d` folder. The only remaining
settings inside the root `.buckconfig` should be setting specific to that
project.

This refactor also applies a new naming scheme for the root cell of Meta
projects. By deriving the root cell name from the public github repo, it makes
it easier to import external cells and avoid resolution errors due to shared
cell names. Moving forward, the expectation is that "root" shall be a cell
alias that always refers to the current cell, so each cell may always trust
that by using "root//" they are staying within their code (which was not a
guarantee before)

Reviewed By: bigfootjon

Differential Revision: D64579239

fbshipit-source-id: cb063176457d458bea4cc8891c4df323b778246b

.buckconfig.d/common.buckconfig

@@ -0,0 +1,20 @@
+[cells]
+prelude = prelude
+none = none
+
+[cell_aliases]
+config = prelude
+ovr_config = prelude
+bazel_skylib = gh_facebook_buck2_shims_meta
+buck = gh_facebook_buck2_shims_meta
+fbcode = gh_facebook_buck2_shims_meta
+fbcode_macros = gh_facebook_buck2_shims_meta
+fbsource = gh_facebook_buck2_shims_meta
+shim = gh_facebook_buck2_shims_meta
+toolchains = gh_facebook_buck2_shims_meta
+
+[external_cells]
+prelude = bundled
+
+[parser]
+target_platform_detector_spec = target:root//...->prelude//platforms:default target:shim//...->prelude//platforms:default

shim/.buckconfig

@@ -6,22 +6,33 @@
 # of this source tree.
 
 [cells]
-shim = .
+gh_facebook_buck2_shims_meta = .
+
+[cell_aliases]
+root = gh_facebook_buck2_shims_meta
+
+# This is a duplicate of .buckconfig.d/common.buckconfig, but due to the way
+# we vendor the shim directory, common.buckconfig won't show up in the
+# vendored .buckconfig.d directory.
+# When the shim cell is no longer vendored in individual projects, this
+# duplication cam be removed.
+[cells]
 prelude = prelude
-toolchains = .
 none = none
 
 [cell_aliases]
 config = prelude
 ovr_config = prelude
-fbcode = shim
-fbsource = shim
-fbcode_macros = shim
-bazel_skylib = shim
-buck = shim
+bazel_skylib = gh_facebook_buck2_shims_meta
+buck = gh_facebook_buck2_shims_meta
+fbcode = gh_facebook_buck2_shims_meta
+fbcode_macros = gh_facebook_buck2_shims_meta
+fbsource = gh_facebook_buck2_shims_meta
+shim = gh_facebook_buck2_shims_meta
+toolchains = gh_facebook_buck2_shims_meta
 
 [external_cells]
 prelude = bundled
 
 [parser]
-target_platform_detector_spec = target:shim//...->prelude//platforms:default
+target_platform_detector_spec = target:root//...->prelude//platforms:default target:shim//...->prelude//platforms:default

shim/build_defs/lib/oss.bzl

@@ -40,11 +40,11 @@ def _strip_third_party_rust_version(target: str) -> str:
 # Cell the BUCK file being processed belongs to
 ACTIVE_CELL = native.get_cell_name()
 
-# The root cell of this project, generally "root" and does not need to be set.
+# The root cell of this project, extracted from the "root" alias.
 # Targets that explicitly reference this cell will not be rewritten, and
 # targets that do not end up referencing a cell will be replaced with targets
 # that reference this cell
-ROOT_CELL = read_config("oss", "root_cell", "root")
+ROOT_CELL = read_config("cell_aliases", "root", "root")
 
 # The cell this file and the rest of the shim directory belong to, generally
 # "shim" and does not need to be set.

shim/shims.bzl

@@ -53,8 +53,8 @@ CPP_UNITTEST_DEPS = [
     "shim//third-party/googletest:cpp_unittest_main",
 ]
 CPP_FOLLY_UNITTEST_DEPS = [
-    "folly//folly/test/common:test_main_lib",
-    "folly//folly/ext/buck2:test_ext",
+    "gh_facebook_folly//folly/test/common:test_main_lib",
+    "gh_facebook_folly//folly/ext/buck2:test_ext",
 ]
 
 def _get_headers_from_sources(srcs):