WIP: [PoC] fx.Evaluate

[abhinav]

This is just a PoC at this time and not yet ready for review.

Resolves #1132

Details

This change introduces the fx.Evalute option,
which allows for dynamic generation of provides, invokes, decorates,
and other evaluates based on the state of existing dependencies.

It addresses one of the more significant remaining pain points
with usage of Fx in large applications and ecosystems:
the ability to programmatically generate parts of the dependency graph.

Concretely, this makes the following possible:

fx.Evaluate(func(cfg *Config) fx.Option {
    if cfg.Environment == "production" {
        return fx.Provide(func(*sql.DB) Repository {
            return &sqlRepository{db: db}
        }),
    } else {
        return fx.Provide(func() Repository {
            return &memoryRepository{}
        })
    }
}),
fx.Provide(func(...) *sql.DB { ... }),

With fx.Evaluate, the dependency on *sql.DB is present in the graph
only in production environments.
In development environments, the dependency connection is absent,
and therefore the database connection is never established.

(Today, attempting to write an Fx module that switches on the backend
like this will result in the SQL connection being established even in
development environments.)

TODO

• Decide whether nilling out the slice is the right approach
  for tracking what's been already run
• fx.Module handling
• fx.WithLogger handling
• Testing with every other feature
• Documentation

[abhinav]

This change is part of the following stack:

• WIP: [PoC] fx.Evaluate #1259 ◀

_{Change managed by git-spice.}

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 29 lines in your changes missing coverage. Please review.

Project coverage is 97.81%. Comparing base (0ad8a04) to head (045ba96).

Files with missing lines	Patch %	Lines
evaluate.go	61.66%	20 Missing and 3 partials ⚠️
app.go	57.14%	2 Missing and 1 partial ⚠️
module.go	85.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1259      +/-   ##
==========================================
- Coverage   98.62%   97.81%   -0.82%     
==========================================
  Files          35       36       +1     
  Lines        3337     3427      +90     
==========================================
+ Hits         3291     3352      +61     
- Misses         39       63      +24     
- Partials        7       12       +5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.