data-source/aws_lambda_function: Return unqualified arn attribute by default, properly return error for missing function

[bflad]

Builds on #5812
Closes #6966
Additional reference: hashicorp/terraform#10810 (comment)

Previously, the aws_lambda_function data source was utilizing the Read function from the aws_lambda_function resource. This legacy practice has longterm maintenance issues with missing schema and documentation updates.

Here we implement a fresh new Read function for the data source that includes the following changes:

• Properly error when Lambda Function is not found
• Always return the arn attribute as unqualified (e.g. without a qualifier or version suffix)
• Always return the qualified_arn attribute as qualified (e.g. with the qualifier or version suffix)
• Always return the tags attribute

Configuration workarounds such as the following can now be removed:

# This configuration contains previous workarounds, which can be removed after upgrading to version 2.0.0 of the Terraform AWS Provider

# This workaround was used to ensure the arn attribute was never qualified
data "aws_lambda_function" "workaround1" {
  function_name = "example"
  qualifier     = ""
}

# This workaround was used to manually fix the arn attribute which was qualified with :$LATEST previously
data "aws_lambda_function" "workaround2" {
  function_name = "example"
}

output "unqualified_arn" {
  value = "${replace(data.aws_lambda_function.workaround2.arn, ":$LATEST", "")}"
}

The acceptance testing changes modernize and simplify the testing:

• Utilize resource.TestCheckResourceAttrPair() where possible to ensure data source state values match appropriate resource state values
• Consolidate random naming to single variable
• Only provision VPC resources in VPC specific test

Output from acceptance testing:

--- PASS: TestAccDataSourceAWSLambdaFunction_version (20.89s)
--- PASS: TestAccDataSourceAWSLambdaFunction_environment (22.75s)
--- PASS: TestAccDataSourceAWSLambdaFunction_alias (23.68s)
--- PASS: TestAccDataSourceAWSLambdaFunction_basic (23.76s)
--- PASS: TestAccDataSourceAWSLambdaFunction_layers (28.82s)
--- PASS: TestAccDataSourceAWSLambdaFunction_vpc (36.48s)

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