Previously the graph builders all expected to be given a full manifest
of all of the plugin component schemas that they could need during their
analysis work. That made sense when terraform.NewContext would always
proactively load all of the schemas before doing any other work, but we
now have a load-as-needed strategy for schemas.
We'll now have the graph builders use the contextPlugins object they each
already hold to retrieve individual schemas when needed. This avoids the
need to prepare a redundant data structure to pass alongside the
contextPlugins object, and leans on the memoization behavior inside
contextPlugins to preserve the old behavior of loading each provider's
schema only once.
The responsibility for actually instantiating a single plugin and reading
out its schema now belongs to the contextPlugins type, which memoizes the
results by each plugin's unique identifier so that we can avoid retrieving
the same schemas multiple times when working with the same context.
This doesn't change the API of Context.Schemas but it does restore the
spirit of an earlier version of terraform.Context which did all of the
schema loading proactively inside terraform.NewContext. In an earlier
commit we reduced the scope of terraform.NewContext, making schema loading
a separate step, but in the process of doing that removed the effective
memoization of the schema results that terraform.NewContext was providing.
The memoization here will play out in a different way than before, because
we'll be treating each plugin call as separate rather than proactively
loading them all up front, but this is effectively the same because all
of our operation methods on Context call Context.Schemas early in their
work and thus end up forcing all of the necessary schemas to load up
front nonetheless.
In the v0.12 timeframe we made contextComponentFactory an interface with
the expectation that we'd write mocks of it for tests, but in practice we
ended up just always using the same "basicComponentFactory" implementation
throughout.
In the interests of simplification then, here we replace that interface
and its sole implementation with a new concrete struct type
contextPlugins.
Along with the general benefit that this removes an unneeded indirection,
this also means that we can add additional methods to the struct type
without the usual restriction that interface types prefer to be small.
In particular, in a future commit I'm planning to add methods for loading
provider and provisioner schemas, working with the currently-unused new
fields this commit has included in contextPlugins, as compared to its
predecessor basicComponentFactory.
