Workspace cache continuously invalidated by IDE metadata files

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/875)
In GitLab by [[Gitlab user @alex-broad]](https://gitlab.com/alex-broad) on Jan 18, 2019, 11:02

## Summary

[//]: Workspace cache continuously invalidated by IDE metadata files

## Steps to reproduce

[//]: Start a project from existing source code with IntelliJ IDEA; point it at the workspace directory and accept the default to write the /.idea (directory based) and *.iml files to the workspace directory. Switch between the IDE (just to look at the code) and build the element. Observe the cache-breaking behaviour.

## What is the current bug behavior?

[//]: The cache gets invalidated and sometimes and/or the build fails and has to be re-run. It can be severe enough that the only way to reliably have it work is to close the IDE before doing `bst build`.

## What is the expected correct behavior?

[//]: Ability to specify files to be ignored a la `.gitignore` (perhaps with a separate `.bstignore` that can reference/import `.gitignore`?)

## Relevant logs and/or screenshots

[//]: #

## Possible fixes

[//]: # Ability to specify files to be ignored a la `.gitignore` (perhaps with a separate `.bstignore` that can reference/import `.gitignore`?)

## Other relevant information

[//]: # (Add the BuildStream version. Substitute below the "x" by "1", "2" or the right version. Add more than one version if necessary)

* BuildStream version affected: /milestone %BuildStream_v1.2.3

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Start by reproducing the issue with an IntelliJ IDEA workspace containing /.idea and *.iml files, then run the affected `bst build` workflow and trace how workspace files affect cache invalidation. The change is done when configurable ignored files, potentially through a separate `.bstignore`, prevent IDE metadata from invalidating the workspace cache or breaking the build.

Written by the indexing model from the issue text.