Check allowed subprojects & environments separately instead of via parser generation

[isomorpheme]

Currently, the configured subprojects and environments are essentially baked into the command parser:

hoff/src/Parser.hs

Line 248 in e9d0cb2

pSubproject = P.choice (fmap P.string subprojects)

hoff/src/Parser.hs

Line 286 in e9d0cb2

pDeployEnvironments = map (fmap DeployEnvironment . P.string) environments

If the subprojects to deploy are instead parsed as arbitrary identifiers, and then checked later, we can do much better error reporting, i.e. including the list of configured subprojects. And that makes the parser more pure in a sense, because it now no longer has to depend on the project config.

[robbert-vdh]

See my comment in #270 (comment). The error messages originally included all of the possible options, but this broke in later additions made to the parser. I think getting the old behavior back for both the deployment environments and the subprojects shouldn't involve much more than shuffling a couple parentheses and possibly whitespace consumptions around.