Expose `prepare.distgit`

[LecrisUT]

Working a bit on refactoring cmake's tmt I've come to the situation where upstream does not have a tmt setup, but still I want to get the exact source (and test data) used for the build (including patching and all), and run the integrated tests. For this exposing prepare.how = distgit would be quite useful as a shortcut, with some tweaks.

• phase_name becomes optional
• add dist_git_type to the prepare's data
• expose the srpm on TMT_PLAN_DATA
• where should the extracted source go? Maybe TMT_SOURCE_DIR should be available more generally?

[lukaszachy]

* expose the `srpm` on `TMT_PLAN_DATA`

Do we want to build srpm? Would you mind illustrating the use case?

[LecrisUT]

Not to build the srpm, but to have the srpm file and the extracted form available. Having the SRPM might come in handy if the sources are something other than Source0. As for the main use-case, let's take cmake:

• Cmake has a testsuite that can be run against pre-installed versions by pointing to the install prefix
• This testsuite is not exposed by the package so the only place to get it from is the srpm (such that it is linked with the version installed)
• I want to get this testsuite in a prepare step so that I can define different tests pointing to the expanded srpm
  • A set of tests is to run the testsuite itself, but I want them as separate tests to more easily navigate
  • A special test is to actually run the rpm build from the srpm but with --with bootstrap. This pattern I also need for some rust packages because their testsuite is disabled because of circular dependency