fix: matchspec build / version from brackets and string serialization

[wolfv]

We were parsing brackets correctly, but we overrode it with None.

This fixes matchspecs like foo * [build="*bla*"]

For now this is no error when the build string is set twice (e.g. foo * bar*[build="baz"]). I am not sure if we should treat that as an error (at least in strict mode?). Same goes for the version.

cc @jaimergp thanks for discovering this today!

[wolfv]

Interestingly in conda, it looks like brackets are always preferred (which is the inverse of what I did here).

For example:

>>> a = MatchSpec("foo [version=5.5.5] 1.2.3 foo")
>>> a
MatchSpec("foo==5.5.5=foo")

We can easily match conda behavior here. We could also add the option to fail on this in strict mode and allow version / build only once in strict mode (or only allow duplication if the value is exactly the same?).

[baszalmstra]

That seems like a good approach!

[wolfv]

I implemented that strict parsing fails when encountering a key multiple times (in brackets, or outside brackets & inside brackets).
IMO this is good to go! :)