Improve customizability for downstream projects

The osname ("apertis" in our case) is used mainly in three places:

1. the packaging branch prefix like apertis/v2020
2. the packaging tag prefix, like apertis/1.42
3. the OBS project for the upload, like apertis:v2020:target

This MR is meant to address 2, while point 3 was addressed by commit 1d04a67d.

This MR also partially addresses point 1,with the final fix blocked by the upstream issue https://gitlab.com/gitlab-org/gitlab/-/issues/35438.

ci-package-builder.yml

 # vi:et:sts=2
 
 variables:
+  OSNAME: apertis
+  # we cannot just stick the regex in a variable due to
+  # https://gitlab.com/gitlab-org/gitlab/-/issues/35438
+  OSNAME_BRANCH_RULE: &osname_branch_rule $CI_COMMIT_BRANCH =~ /^apertis\//
+  OBS_REPOSITORIES: default
+
   NAME: ${GITLAB_USER_NAME}
   FULLNAME: ${GITLAB_USER_NAME}
   EMAIL: ${GITLAB_USER_EMAIL}
@@ -11,7 +17,6 @@ variables:
   # Disable shallow clone as history is needed for describing tags
   GIT_DEPTH: 0
   LICENSE_BLACKLIST_target: "GPL-3 GPL-3+ AGPL-3 AGPL-3+ AGPL-1 AGPL-1+ LGPL-3 LGPL-3+ BSD-4-Clause MPL-1.1"
-  OBS_REPOSITORIES: default
   # Set OBS_PREFIX in the project package variables to force uploads to a
   # branch. For instance, by setting it to home:yourusername:branches uploads
   # for apertis:v2020:target will end up in home:yourusername:branches:apertis:v2020:target
@@ -127,7 +132,7 @@ build-source:
   tags:
     - lightweight
   script:
-    - TAG=$(git describe --tags --match "apertis/*" --exact || true)
+    - TAG=$(git describe --tags --match "$OSNAME/*" --exact || true)
     - if [ -n "$TAG" ]; then echo "Skip building the package, already tagged as '$TAG'"; exit 0; fi
     - ci-buildpackage -S --git-prebuild='debian/rules debian/control || true'
     - mv _build/ _build-snapshot/
@@ -204,16 +209,16 @@ tag-release:
         exit 1
       fi
     - |
-      if git describe --tags --match 'apertis/*' --exact-match > /dev/null 2>&1
+      if git describe --tags --match "$OSNAME/*" --exact-match > /dev/null 2>&1
       then
         echo "Fetching the already tagged source package version for this commit"
         pristine-lfs -v checkout --full --branch ${PRISTINE_SOURCE} --auto -o _build-release/
         exit 0
       fi
-    - VERSIONTAG=apertis/$(dpkg-parsechangelog -SVersion | sed -e 'y/:~/%_/' -e 's/\.\(\.\|$\|lock$\)/.\1#/g')
+    - VERSIONTAG=$OSNAME/$(dpkg-parsechangelog -SVersion | sed -e 'y/:~/%_/' -e 's/\.\(\.\|$\|lock$\)/.\1#/g')
     - echo "debian/changelog points to ${VERSIONTAG}, looking for it"
     - |
-      if git describe --tags --match 'apertis/*' --exact-match ${VERSIONTAG} > /dev/null 2>&1
+      if git describe --tags --match "$OSNAME/*" --exact-match ${VERSIONTAG} > /dev/null 2>&1
       then
         echo "debian/changelog points to ${VERSIONTAG} which already exists, skip"
         exit 0
@@ -223,8 +228,8 @@ tag-release:
         --git-tag-only
         --git-ignore-branch
         --git-ignore-new
-        --git-debian-tag='apertis/%(version)s'
-        --git-debian-tag-msg='%(pkg)s Apertis release %(version)s'
+        --git-debian-tag="$OSNAME/%(version)s"
+        --git-debian-tag-msg="%(pkg)s $OSNAME release %(version)s"
     - ci-buildpackage -S --git-prebuild='debian/rules debian/control || true'
     - mv _build/ _build-release/
     - pristine-lfs -v import-dsc --full --branch "${PRISTINE_SOURCE}" _build-release/*.dsc
@@ -236,7 +241,7 @@ tag-release:
   rules:
     - if: $CI_MERGE_REQUEST_ID
       when: never
-    - if: $CI_COMMIT_BRANCH =~ /^apertis\//
+    - if: *osname_branch_rule
       changes:
         - debian/changelog
       when: on_success
@@ -310,7 +315,7 @@ upload:
             - build.log
           when: always
   script:
-    - git describe --tags --match 'apertis/*' --exact && export RELEASE=1
+    - git describe --tags --match "$OSNAME/*" --exact && export RELEASE=1
     - |
       if [ "$RELEASE" == 1 ]
       then
@@ -369,7 +374,7 @@ upload:
   rules: &upload_rules
     - if: $CI_MERGE_REQUEST_ID
       when: never
-    - if: $CI_COMMIT_BRANCH =~ /^apertis\//
+    - if: *osname_branch_rule
       when: on_success
     - when: never
   dependencies: