Multiple usages of config usages show up in schema

[wesbillman]

If a file has something like this in it:

var hostname = ftl.Config[string]("FTL_ENDPOINT")

// and somewhere else in that package they have

var frontendURL = ftl.Config[string]("FTL_ENDPOINT")

The schema will have duplicates

module echo {
  config FTL_ENDPOINT String

  config FTL_ENDPOINT String

[deniseli]

Continuing on from the above PR:

• Add check for duplicate databases
• Add checks to backend for all 3 types