_version.py is being generated in the wrong place

[zooko]

The buildbot clearly told us that something was wrong with 59dcd5014dcb440a when every builder went red. I really couldn't figure out how the patch, which didn't touch any of the tahoe-lafs code or any of the setup script code could possibly cause this problem.

After much head-scratching I realized that I was looking at the consequences of my own decision to make darcsver too clever by half almost three years ago:

​http://tahoe-lafs.org/trac/darcsver/browser/trunk/darcsver/setuptools_command.py?annotate=blame&rev=109#L74

If the user of darcsver (which is in this case the tahoe-lafs setup.py) doesn't specify into what file(s) the resulting version number should go, then darcsver will guess about where it should go. Before 59dcd5014dcb440a, darcsver was guessing correctly by noticing that all of the contents of the allmydata-tahoe package ("distribution") came from a directory named src/allmydata/, so it decided to put the version number into src/allmydata/_version.py. After 59dcd5014dcb440a, it now guesses it should put the version number into src/_version.py instead since 59dcd5014dcb440a added some files in a new directory named src/buildtest/ which is outside of src/allmydata/.

I'm pretty sure the best fix for this is to make the tahoe-lafs setup.py explicitly specify which file the version number should go into.

In addition, I should probably consider removing the magical cleverness from darcsver so that users who don't bother to specify a filename get a runtime failure instead of a good guess.

[zooko@…]

In e03206cbdf217893:

setup: specify that the version file must go into src/allmydata/_version.py . fixes #1259