update externals format to match existing code

lib/spack/docs/getting_started.rst

@@ -474,9 +474,14 @@ assuming the paths to the compiler executables are determinable from
 the prefix.
 
 If the paths to the compiler executable are not determinable from the
-prefix, you can add them to the ``extra_attributes`` field. Similarly,
-all other fields from the compilers config can be added to the
-``extra_attributes`` field for an external representing a compiler.
+prefix, you can add them to the ``extra_attributes`` field using the
+``compilers`` key. The ``compilers`` key accepts compilers for ``c``,
+``cxx``, ``fortran``, and ``f77``.
+
+For all other fields from the ``compilers`` config, they can be added
+to the ``extra_attributes`` field for an external representing a
+compiler. These fields are used as-is in the internal representation
+of the compiler config.
 
 .. code-block:: yaml
 
@@ -493,11 +498,10 @@ all other fields from the compilers config can be added to the
        - spec: llvm+clang@15.0.0 arch=linux-rhel8-skylake
          prefix: /usr
          extra_attributes:
-           paths:
-             cc: /usr/bin/clang-with-suffix
+           compilers:
+             c: /usr/bin/clang-with-suffix
              cxx: /usr/bin/clang++-with-extra-info
-             fc: /usr/bin/gfortran
-             f77: /usr/bin/gfortran
+             fortran: /usr/bin/gfortran
            extra_rpaths:
            - /usr/lib/llvm/
 

lib/spack/spack/compilers/__init__.py

@@ -167,7 +167,17 @@ def _compiler_config_from_external(config):
     prefix = config.get("prefix", None)
 
     compiler_class = class_for_compiler_name(compiler_spec.name)
-    paths = extra_attributes.get("paths", {})
+    paths = extra_attributes.get("compilers", {})
+
+    # compilers format has cc/fc/f77, externals format has "c/fortran"
+    if "c" in paths:
+        paths["cc"] = paths.pop("c")
+    if "fortran" in paths:
+        fc = paths.pop("fortran")
+        paths["fc"] = fc
+        if "f77" not in paths:
+            paths["f77"] = fc
+
     compiler_langs = ["cc", "cxx", "fc", "f77"]
     for lang in compiler_langs:
         if paths.setdefault(lang, None):

lib/spack/spack/test/cmd/compiler.py

@@ -259,11 +259,10 @@ def test_compiler_list_empty(no_compilers_yaml, working_env, compilers_dir):
                 "prefix": "/path/to/fake",
                 "modules": ["gcc/7.7.7", "foobar"],
                 "extra_attributes": {
-                    "paths": {
-                        "cc": "/path/to/fake/gcc",
+                    "compilers": {
+                        "c": "/path/to/fake/gcc",
                         "cxx": "/path/to/fake/g++",
-                        "fc": "/path/to/fake/gfortran",
-                        "f77": "/path/to/fake/gfortran",
+                        "fortran": "/path/to/fake/gfortran",
                     },
                     "flags": {"fflags": "-ffree-form"},
                 },