glibc: provides iconv (#43897)

`iconv` is a bit of weird virtual because the only shared API between
`glibc` and `libiconv` is:

```
iconv
iconv_open
iconv_close
```

whereas `libiconv` has further symbols [iconvctl](https://www.gnu.org/software/libiconv/documentation/libiconv-1.17/iconvctl.3.html), [iconv_open_into](https://www.gnu.org/software/libiconv/documentation/libiconv-1.17/iconv_open_into.3.html), and an `iconv` executable and `libcharset.so`. Packages that need those will have to do `depends_on("[virtuals=iconv] libiconv")`.

etc/spack/defaults/cray/packages.yaml

@@ -0,0 +1,19 @@
+# -------------------------------------------------------------------------
+# This file controls default concretization preferences for Spack.
+#
+# Settings here are versioned with Spack and are intended to provide
+# sensible defaults out of the box. Spack maintainers should edit this
+# file to keep it current.
+#
+# Users can override these settings by editing the following files.
+#
+# Per-spack-instance settings (overrides defaults):
+#   $SPACK_ROOT/etc/spack/packages.yaml
+#
+# Per-user settings (overrides default and site settings):
+#   ~/.spack/packages.yaml
+# -------------------------------------------------------------------------
+packages:
+  all:
+    providers:
+      iconv: [glibc, musl, libiconv]

etc/spack/defaults/linux/packages.yaml

@@ -0,0 +1,19 @@
+# -------------------------------------------------------------------------
+# This file controls default concretization preferences for Spack.
+#
+# Settings here are versioned with Spack and are intended to provide
+# sensible defaults out of the box. Spack maintainers should edit this
+# file to keep it current.
+#
+# Users can override these settings by editing the following files.
+#
+# Per-spack-instance settings (overrides defaults):
+#   $SPACK_ROOT/etc/spack/packages.yaml
+#
+# Per-user settings (overrides default and site settings):
+#   ~/.spack/packages.yaml
+# -------------------------------------------------------------------------
+packages:
+  all:
+    providers:
+      iconv: [glibc, musl, libiconv]

var/spack/repos/builtin/packages/glibc/package.py

@@ -29,6 +29,7 @@ class Glibc(AutotoolsPackage, GNUMirrorPackage):
     license("LGPL-2.1-or-later")
 
     provides("libc")
+    provides("iconv")
 
     version("master", branch="master")
     version("2.39", sha256="97f84f3b7588cd54093a6f6389b0c1a81e70d99708d74963a2e3eab7c7dc942d")

var/spack/repos/builtin/packages/musl/package.py

@@ -34,6 +34,7 @@ class Musl(MakefilePackage):
     representative_headers = ["iso646.h"]
 
     provides("libc")
+    provides("iconv")
 
     version("1.2.4", sha256="7a35eae33d5372a7c0da1188de798726f68825513b7ae3ebe97aaaa52114f039")
     version("1.2.3", sha256="7d5b0b6062521e4627e099e4c9dc8248d32a30285e959b7eecaa780cf8cfd4a4")