CSP fixer: shorter progress message
CSP fixer: ignore special dirs when processing (in particular .bower-git, .git)
CSP fixer: don't create backups of original sources by default

1) They are largely useless during normal development:
- CSP fixing is typically applied to vendored sources, which can be recovered in other ways;
- user's own sources can be typically recovered from source control;
- even if the above doesn't apply, the source can be reconstructed from the newly generated parts with some manual work.
2) Creating backups polutes the disk and slows down CSP fixing.
CSP Fixer: throttle concurrent fixing threads to unfreeze the UI

This is a stop-gap measure; a proper fix should be using a ServiceWorker. TODO added.
Address comments
Merge pull request #3805 from ussuri/csp-fixer-improvements

CSP fixer improvements
Added `.bower-git` to isScmPrivate + removed outdated `cache`
Merge pull request #3810 from ussuri/update-isScmPrivate

Added `.bower-git` to isScmPrivate + removed outdated `cache`
Merge remote-tracking branch 'upstream/master' into bower-fixes
- Added PackageServiceProperties.getMatchingFolderWithPackages(), to work in tandem with .isPackageResource.
- Used it on the UI side (spark.dart) to extract the folder with packages for a resource for which isPackageResource() returned true.
- With that, the UI no longer has any logic about what resource the get/update packages action is aplicable to, and what the matching folder with package is - all of that is concentrated in PackageServiceProperties(), so it become self-consistent.
- With that, now always pass the top-level folder with packaged to getOrUpgradePackages() in both Pub and Bower.
- As a result, getOrUpgradePackages in both became less convoluted and Pub lost _getPackageDir().