Write a new schema only if necessary

[iwahbe]

As I explain in my change:

For the Schema language in particular, we only write files if the write
would cause a change. This allows build systems like Make to correctly
include bridge-metadata.json and schema.json as build dependencies
without always trying to rebuild.

We don't do this for all systems because reading every file before writing it is
expensive.

I've been playing around with using real file dependencies for Go and Make, and the way we always write bridge metadata (which is then a build input via go:embed) prevents Make from ever observing that a provider is already built.

[codecov]

Codecov Report

Attention: Patch coverage is 62.06897% with 11 lines in your changes missing coverage. Please review.

Project coverage is 67.73%. Comparing base (a3a15d2) to head (cde62c6).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/tfgen/generate.go	62.06%	7 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2877      +/-   ##
==========================================
- Coverage   67.74%   67.73%   -0.02%     
==========================================
  Files         328      328              
  Lines       42088    42116      +28     
==========================================
+ Hits        28513    28526      +13     
- Misses      11987    11999      +12     
- Partials     1588     1591       +3     

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[VenelinMartinov]

lgtm

[danielrbradley]

@iwahbe I think this might have the unintened concequence of leaving the schema as always being out of date. If the timestamp on the emitted file is still before that of the source files, then the next time the build is run, it will still have to generate the schema, only to compare it to what's on disk again. In the make world, timestamps of completed file targets always need to be updated even if the file hasn't changed.

I would suggest this might need to be reverted 🫤

[pulumi-bot]

This PR has been shipped in release v3.103.0.