Add %crate_instdir convenience macro

Fixes #41.

The template is adjusted to use the new macro. This will only work
if the macro is defined, thus the required version of rust-packaging
is bumped.

data/macros.cargo

@@ -10,6 +10,8 @@
 
 %cargo_registry %{_datadir}/cargo/registry
 
+%crate_instdir %{cargo_registry}/%{crate}-%{version_no_tilde}
+
 %__cargo_is_lib() %__cargo_inspector --target-kinds Cargo.toml | grep -q -F -x "$(printf 'lib\\\nrlib\\\nproc-macro')"
 %__cargo_is_bin() %__cargo_inspector --target-kinds Cargo.toml | grep -q -F -x bin
 

rust2rpm/templates/main.spec

@@ -43,7 +43,7 @@ Patch0:         {{ patch_file }}
 
 ExclusiveArch:  %{rust_arches}
 
-BuildRequires:  rust-packaging
+BuildRequires:  rust-packaging >= 21
 {% if all_features %}
   {% set cargo_args = " -a" %}
 {% endif %}
@@ -160,9 +160,9 @@ use {% if feature is not none %}the "{{ feature }}" feature of {% endif %}the "%
       {% if license_files|length > 0 %}
 %license {{ license_files|join(' ') }}
       {% endif %}
-%{cargo_registry}/%{crate}-%{version_no_tilde}/
+%{crate_instdir}/
     {% else %}
-%ghost %{cargo_registry}/%{crate}-%{version_no_tilde}/Cargo.toml
+%ghost %{crate_instdir}/Cargo.toml
     {% endif %}
 
   {% endfor %}