Create custom Xcode schemes

[cgrindel]

Related to #573.

• Add initializers with defaults to XcodeScheme structs.
• Add XcodeScheme.ProfileAction.
• Move shouldCreateScheme filter to createAutogeneratedXCSchemes. It was in the shared code.
• Add targetResolver(referencedContainer:) to Fixtures.

[brentleyjones]

If we are creating custom schemes, shouldn't the ones for Generator have changed?

[cgrindel]

@brentleyjones You are correct. I will investigate before merging this PR.

Not done

[cgrindel]

The //tools/generator:xcodeproj target is configured with scheme_autogeneration_mode set to all. Hence, it is generating both the custom and autogenerated scheme. The custom scheme is named Generator. One of the autogenerated schemes is called generator. The last one in the list wins.

For this PR, I will rename the custom scheme to not conflict. I have added two tasks to #573.

• Enable scheme_autogenrate_mode auto.
• Ensure that autogenerated scheme names do not conflict with custom scheme names.

[cgrindel]

I verified that the custom scheme is written (Good catch, @brentleyjones). I launched the scheme in Xcode and was able to build and test it successfully.

[brentleyjones]

When there is conflicting schemes, can we have the custom ones just override the automatic one? Seems better than an error, since one controls the name of the custom scheme.

[brentleyjones]

(Would require changing the order that they are made in, but I think that's for the best. Or the automatic ones can get a list of custom ones made and not make ones from it, etc.)

[cgrindel]

When there is conflicting schemes, can we have the custom ones just override the automatic one? Seems better than an error, since one controls the name of the custom scheme.

Sure. I was going to add code that checks an autogenerated scheme name and add a prefix (e.g., (generated)), if there was a conflict. However, I am happy to just skip the autogenerated one. Whichever you want is fine with me.

[brentleyjones]

I like skipping the autogenerated one. Then one can use top_level and still define a custom one to "tweak" and existing top level autogenerated scheme.