Emit ProviderAlreadyInstalled when provider installed

[pselle]

Emit the ProviderAlreadyInstalled event when we successfully verify that we've already installed this provider and are skipping installation.

Before:

$ terraform init

Initializing the backend...

Initializing provider plugins...
- Reusing previous version of providerland/foo from the dependency lock file

Terraform has been successfully initialized!

After:

$ terraform init

Initializing the backend...

Initializing provider plugins...
- Reusing previous version of providerland/foo from the dependency lock file
- Using previously-installed providerland/foo v0.0.8

Terraform has been successfully initialized!

[codecov]

Codecov Report

Merging #27722 (aa24bfe) into master (96be094) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Impacted Files	Coverage Δ
internal/providercache/installer.go	68.54% <100.00%> (+0.29%)	⬆️

[ghost]

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