Implement new required 'version' specification in project.conftristan/bst2-separation

This adds a required field to project.conf to specify the BuildStream
version required by the project, and this serves two separate purposes
at once:

  A.) It is a more practical method for users to specify the format-version

      The "format-version" field requires that users know about which
      format version each YAML feature was added in, and this version
      is separate from the BuildStream version.

      As such, it is more difficult to use - the field is still
      preserved and supported, but mostly only useful in order
      to depend on format versions in unstable release periods.

  B.) It serves to ensure that the wrong major point version
      of BuildStream is never used with the wrong project.

Changes in this commit include:

  * _versions.py: Added BST_API_VERION_MAJOR and BST_API_VERION_MINOR

    These include comments about how the fields must be updated when
    work commences on any major or minor point stable release, usually
    on the master branch.

  * _project.py: Support parsing the new "version"

  * _frontend/app.py: The `bst init` command has been extended to also
    dump the current API version automatically.

    This should really be enhanced separately, but since the test cases
    use `bst init` in some places in order to create projects for test
    purposes, I've included the minimal change here and will enhance
    `bst init` separately.

  * tests: All tests updated to include a "version: 2.0" specification
    in the project.conf files under test

1 files changed, 2 insertions, 2 deletions

diff --git a/tests/format/include.py b/tests/format/include.py
index 4b1da920b..1143bd4e2 100644
--- a/tests/format/include.py
+++ b/tests/format/include.py
@@ -32,7 +32,7 @@ def test_include_project_file(cli, datafiles):
 
 
 def test_include_missing_file(cli, tmpdir):
-    tmpdir.join('project.conf').write('{"name": "test"}')
+    tmpdir.join('project.conf').write('{"name": "test", "version": "2.0"}')
     element = tmpdir.join('include_missing_file.bst')
 
     # Normally we would use dicts and _yaml.dump to write such things, but here
@@ -51,7 +51,7 @@ def test_include_missing_file(cli, tmpdir):
 
 
 def test_include_dir(cli, tmpdir):
-    tmpdir.join('project.conf').write('{"name": "test"}')
+    tmpdir.join('project.conf').write('{"name": "test", "version": "2.0"}')
     tmpdir.mkdir('subdir')
     element = tmpdir.join('include_dir.bst')