[vcpkg] Fix regression in error messages with registries/versioning

[ras0219]

With the versioning and registries PRs landing, a regression was introduced where syntax issues with builtin ports were no longer displayed.

Before:

PS C:\src\vcpkg> .\vcpkg.old.exe install simage --dry-run
Computing installation plan...
Error: while loading port `simage`: Port definition not found

After:

PS C:\src\vcpkg> .\vcpkg.new.exe install simage --dry-run
Computing installation plan...
Error: There are invalid fields in the control or manifest file of simage
The following fields were not expected:
    In CONTROL: ersion
This is the list of valid fields for CONTROL files (case-sensitive):

    Source
    Version
    Port-Version
    Description
    Maintainer
    Build-Depends
    Homepage
    Type
    Supports
    Default-Features

You may need to update the vcpkg binary; try running .\bootstrap-vcpkg.bat to update.

Error: There are missing fields in the control file of simage
The following fields were missing:
    In CONTROL: Version
Error: failed to load port from C:\src\vcpkg\ports\simage

In addition to fixing this regression, this PR cleans up a few layers of duplicate code or impedance mismatch between APIs (such as loading and parsing from disk the same files multiple times). Especially, this implements PathsPortfileProvider entirely in terms of the new providers.