7z-sys-provided: let user override paths

let the user override assumed paths via environment.
EXTRACTCODE_7Z_PATH is used to determine the path of the 7z
executable.
If not specified it falls back to previously used probe of
distro data.

With this patch it possible to run scancode is buildsystems like YOCTO,
which heaviily rely on overriding paths via environment to pick the
correct implementation from the build workspace

Signed-off-by: Konrad Weihmann <kweihmann@outlook.com>

Author: priv-kweihmann

builtins/extractcode_7z_system_provided/README.rst

@@ -1 +1,5 @@
-A ScanCode Toolkit plugin to use pre-installed system binary libraries and utilities.
+A ScanCode Toolkit plugin to use pre-installed 7zip executables
+===============================================================
+
+the path of 7zip is either determined by distro data or explicitily
+taken from ``EXTRACTCODE_7Z_PATH`` environment variable

builtins/extractcode_7z_system_provided/src/extractcode_7z/__init__.py

@@ -9,7 +9,9 @@
 
 
 import platform
+from os import environ
 from os import path
+from os import pathsep
 
 from plugincode.location_provider import LocationProviderPlugin
 
@@ -22,26 +24,31 @@ def get_locations(self):
         locations of the 7zip exe and shared libraries as installed on various
         Linux distros or on FreeBSD.
         """
-        mainstream_system = platform.system().lower()
-        if mainstream_system == 'linux':
-            distribution = platform.linux_distribution()[0].lower()
-            debian_based_distro = ['ubuntu', 'mint', 'debian']
-            rpm_based_distro = ['fedora', 'redhat']
-
-            if distribution in debian_based_distro:
-                lib_dir = '/usr/lib/p7zip'
-
-            elif distribution in rpm_based_distro:
-                lib_dir = '/usr/libexec/p7zip'
-
-            else:
-                raise Exception('Unsupported system: {}'.format(distribution))
-        elif mainstream_system == 'freebsd':
-            lib_dir = '/usr/local/libexec/p7zip'
+        lib_7z = environ.get('EXTRACTCODE_7Z_PATH')
+        if not lib_7z:
+            mainstream_system = platform.system().lower()
+            if mainstream_system == 'linux':
+                distribution = platform.linux_distribution()[0].lower()
+                debian_based_distro = ['ubuntu', 'mint', 'debian']
+                rpm_based_distro = ['fedora', 'redhat']
+
+                if distribution in debian_based_distro:
+                    lib_dir = '/usr/lib/p7zip'
+
+                elif distribution in rpm_based_distro:
+                    lib_dir = '/usr/libexec/p7zip'
+
+                else:
+                    raise Exception('Unsupported system: {}'.format(distribution))
+            elif mainstream_system == 'freebsd':
+                lib_dir = '/usr/local/libexec/p7zip'
+            lib_7z = path.join(lib_dir, '7z')
+        else:
+            lib_dir = path.dirname(lib_7z)
 
         locations = {
             'extractcode.sevenzip.libdir': lib_dir,
-            'extractcode.sevenzip.exe': path.join(lib_dir, '7z'),
+            'extractcode.sevenzip.exe': lib_7z,
         }
 
         return locations