The ParsePluginSource function can be invoked from either a HCL2 context
(when parsing a required_plugins block), or from the command-line
itself.
While in the first context a hcl.Diagnostics is coherent, in case the
source to parse is a command-line argument, for example when installing
or removing a plugin, the error message cannot have an HCL context,
leading to errors that are incorrectly prefixed by a <nil> string dure
to the lack of a reference to attach the diagnostic to.
Therefore, in order to fix this behaviour, the logic that parses plugin
sources now returns an error, and attaching the error to an HCL subject
is done independently, if needed.
Lucas Bajolet
c7f2508373